Here are a few things that cause the car to underbrake (or cause the driver to believe that has happened)
The car was going faster than datum.
Slipstreaming can cause this or an unexpectedly good exit from the corner before. If you have your BP down to within a foot or so and then arrive 10mph faster that will make a difference. Your braking hasn't changed but your perception of it has. Let's be honest, few people look at their speedos while braking.
You have ABS on and bite into the ABS too much.
ABS in real-life is about 30% less efficent than full-retardation braking. It makes up for this by being ON for the whole braking distance. In GT5 you need to keep into the white and out of the red (nothing in this game for two in a bed).
The car was not straight when you applied the brakes.
Tyres work best longtitudinally, especially when braking. If the car is not straight and especially if the car is sliding slightly your braking effect can be significantly reduced. Brake straight, turn, power when you're SURE you can keep it on.
Your downshift pattern changed
The rythmn of the downshift is vital - particularly in the more powerful cars. 'Running On' in a high gear actually means that the engine is still 'pushing' the car while the brakes are trying to retard momentous progression. Not good.
Your car was airborne (as suggested by user888)
As posted... if your car is too stiff it might not be sitting against the tarmac when you hit the brakes. Less contact, less mu, less friction, longer braking.
Your tyres are worn.
Worn tyres are less efficient. Still, after a five lap distance your tyres should be okay even if you live in Hull.
