you failed at physics
why would resistance change?
you need to change the equation to:
P = VI, now say "if" the pump is run at 3bar base pressure + 1.5bar of boost (because yolo).. it would require ~11amp at 13.5V to run the Walbro 255.
This means, it will consume 148.5 watts
so put that back into the equation, now it becomes 148.5 = V x I, now as you reduce V, you need to increase I
Let's say we run the pump at 12V, that becoming 148.5 = 12 x I, current now has increased to 12.375AMP
A good real world example, high voltage power lines are in the 10s of thousands.. the reason behind this is to carry lower current, thus less resistance