There are mechanical things that could *possibly, maybe* cause this, that have been discussed, differing fuel pressure, runner distribution, etc.

From an electronic standpoint, there is the possibility of the PCM commanding different fuel volumes for each bank. This is part of the adaptive fuel trim, or 'learning' that the PCM will do. If this is the cause of the fuel difference from bank to bank, it is probably just a symptom of an o2 sensor issue. It is not part of the normal operation of the PCM. You can test this by unplugging the battery for a minute to clear the adaptive memory, and then make a pass.

Hope this helps,

Brian
EFI-Unlimited