Image this scenario.

Put a sauce pan in the kitchen sink, under the faucet. Turn the faucet
on high and let the pan fill and overflow. Use a spoon and start
spooning water out of the pan. No matter how fast you spoon, the pan
stays full and keeps overflowing. That is how your fuel system is
supposed to supply fuel to the injector lines.

Now, turn the faucet way down. Start spooning the water out slowly. If
you go slow enough, the pan stays full. Start spooning full speed and
soon the water level drops down and you don't get full spoons anymore
unless you slow down again and let the pan fill up. That is the problem
you have with your fuel system.

The Bentley test procedure measures how much water is overflowing the
pan. Since the engine can only use so much fuel even at full load, the
Bentley knows how much overflow must be available to meet that
requirement with plenty to spare. 500 cc per 30 seconds is the spec, so
as parts age and the fuel filter gets used up there is still enough flow
for the vehicle to run fine. Bentley page 20.29

Most likely, your fuel tank or fuel filter are partially blocked.

Mark