That is a very interesting question. I can think of 2 guesses:

1) The injector has a built in flyback diode. (I am unaware that any do but I don't know)

2) The injector is more "sensitive" or faster with one polarity.

Hopefully some smart person will chime in with the answer. You might try putting an ohm meter across the injector one way then the other and see if the readings are any different.

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra