The shirt exhibits a undetermined period of time in which the blood flowed downwards and outwards, around the shirt. The curvature of the stomach region would naturally steer the blood to either side of the torso, and this is what ultimately happened.
It has speculated that the murder of Paul Stine actually occurred one block earlier, at the Washington and Maple Street intersection, which is what the Zodiac Killer claimed all along in his October 13th 1969 correspondence: "I am the murderer of the taxi driver over by Washington St + Maple St last night, to prove this here is a blood stained piece of his shirt."
It was considered, that after he had shot the taxicab driver, the vehicle, not in park, motioned forward out of control, so reacting immediately, the Zodiac Killer exited the left rear door and entered the driver side door, depositing blooded fingerprints on the driver side panel with his right hand as he steadied himself. He then clambered over Paul Stine and pushed his lifeless body against the driver side door, to access the pedals, quite possibly stalling the vehicle at some point. He restarted the taxicab, depositing fingerprints on the keys, before coasting the taxicab to its final resting position, outside 3898 Washington Street..
Looking at the blood pattern on the shirt, it is possible that if the Zodiac Killer had Paul Stine pressed up against the driver side door. Paul Stine's arms would have been forced into his body, explaining the lack of blood in the upper left and right region of the shirt. Despite his arms squeezed into his body, the inner lining of the armpit section of the jacket forms a well, or collection point for the blood rounding the chest area, hence the two blotches on either shirt arm. The blood pattern suggests Paul Stine was pressed against the driver side door, but was to some degree tilted to his left side. If he was perfectly straight or tilted to the right side, how could the blood flow on the left side of the shirt (by the red arrow), be not only the same, but even wider than on the right, bearing in mind the wound was anterior to his right ear. All in all, the blood present on the shirt is fairly equal on both sides, until it abruptly stops, just past the mid-point on the left side, as though it was prevented from running any further.
There could be a number of reasons for this, but one is that Paul Stine's lower left arm was wedged across his body, preventing the blood flowing any further in this direction. The other possibility, is that the left side of Paul Stine's torso was pressed into and against the arm rest of the door, which sits at approximately this point down the shirt line. This may explain why the blood traveled no further down this portion of the shirt and lays down further credence for the murder to have occurred prior to the Washington and Cherry Street intersection.
It would be great to hear other views on the blood distribution present on the shirt, giving an alternative pattern of events, as this is by no means conclusive. Any input would be greatly valued. See also:
1. PRESIDIO HEIGHTS-THE FINAL CHAPTER
2. THE KEYS AND WALLET
3. THE TAXICAB GLOVES
4 PRESIDIO HEIGHTS REVISITED
5. THE SILENT MURDER AT PRESIDIO HEIGHTS