However, there may be a far simpler answer - that Zodiac was correct all along. When you examine the wording in the Zodiac phone call, it is easy to see how this could be changed ever so slightly to make perfect sense. The phone message delivered to Nancy Slover from the murderer and detailed in the police report was the one she remembered, but it may not necessarily be the actual message delivered by the killer.
Let us go ahead and change the message from "I wish to report a double murder. If you will go one mile east on Columbus Parkway to a public park, you will find the kids in a brown car. They have been shot by a 9 mm Luger. I also killed those kids last year.... Good-bye" to "I wish to report a double murder. If you will go east and one mile on Columbus Parkway to a public park, you will find the kids in a brown car. They have been shot by a 9 mm Luger. I also killed those kids last year.... Good-bye."
Keeping the identical words, with the addition of the word 'and' in the correct place, changes the entire meaning of the message and now makes perfect sense, in respect to the Zodiac Killer making a phone call and directing police to the scene of the crime. Heading east along Springs Road brings you onto Columbus Parkway. But the section of Columbus Parkway that heads universally north to the crime scene is approximately 1 mile. See here. It is located on the junction with Lake Herman Road. The entire route along Columbus Parkway from Springs Road to the crime scene is 1.5 miles, but the Zodiac is no doubt approximating the distance in his phone message. Yet, the overall narrative of the phrase is now correct, heading east, before joining Columbus Parkway and continuing north to the public park.
The phone message can be slightly altered with no additions whatsoever, just inserting a comma or slight pause in the way it was spoken: "I wish to report a double murder. If you will go east, one mile on Columbus Parkway to a public park, you will find the kids in a brown car. They have been shot by a 9 mm Luger. I also killed those kids last year.... Good-bye." The structure of the statement remains viable with respect to directions from Zodiac's location or the police station.
It is 2.3 miles in totality from the Vallejo police station to Columbus Parkway, but if Zodiac had underestimated the distance, one further option retaining the same order could be "I wish to report a double murder. If you will go one mile east onto Columbus Parkway, to a public park, you will find the kids in a brown car. They have been shot by a 9 mm Luger. I also killed those kids last year.... Good-bye." or "I wish to report a double murder. If you will go one mile east, on Columbus Parkway by a public park, you will find the kids in a brown car. They have been shot by a 9 mm Luger. I also killed those kids last year.... Good-bye."
In fact, the wording in Zodiac's message can remain totally unaltered, yet still contain a totally different meaning to the one widely accepted - if we just insert a comma or slight pause in the way it is spoken. This way, Nancy Slover could have remembered every word in order correctly, but simply failed to interpret the method of delivery: "I wish to report a double murder. If you will go one mile east,.... on Columbus Parkway to a public park, you will find the kids in a brown car. They have been shot by a 9 mm Luger. I also killed those kids last year.... Good-bye"
The payphone is roughly 10 minutes by car from the crime scene, but there was a 40 minute delay after the murder and attempted murder of Darlene Ferrin and Michael Mageau, leading to speculation that the killer may have lived nearby, going home to possibly ditch the murder weapon before returning to place the call. If this were the case, it would surely lend credence to the idea that the Zodiac Killer was all too familiar with his his surroundings, and the phone message to Nancy Slover was simply a case of crossed wires.
The Zodiac Killer, having placed the phone call to the Vallejo Police Department, was clearly directing the police from the station to the crime scene. The quickest route for police leaving the 111 Amador Street police station, was to enter Solano Avenue and join Springs Road, from where they would be heading east towards Columbus Parkway and the crime scene. So, when the Zodiac Killer stated "if you will go one mile east" to the police, he was pretty much correct. When the police joined Springs Road, they would have traveled only 1.4 miles east before entering Columbus Parkway. If the Zodiac Killer hadn't approximated his distance, then the original message to Nancy Slover would have read "I wish to report a double murder. If you will go 1.4 mile east....... on Columbus Parkway to a public park, you will find the kids in a brown car. They have been shot by a 9 mm Luger. I also killed those kids last year.... Good-bye".
What Zodiac investigators and researchers are doing in the case of the payphone message are just blindly assuming that Nancy Slover recalled the message perfectly word-for-word in context, while automatically assuming that the Zodiac Killer got the message wrong. The phone call was comprised of 46 words delivered in approximately 20 seconds, with part talking over one another. It is easy to see how one word, one comma or one full stop can be omitted from the final written and recorded message in the police report. The omission of a full stop between "east" and "on" changes the entire meaning of the message.