Detector rails normally activate for ~20 ticks (depends on how fast the minecart goes) Sometimes though they randomly activate for something that looks like half of that normal time even if the minecart moves with the same speed.
To set up an experiment, create ~ 3x5 loop with some powered rails and a detector rail. Launch an empty minecart. Output the signal from the detector into a line of ~15 repeaters on 1 tick delay. Observe gaps in the output change in size sometimes.
P.S. This might relate to a bug which made a button to sometimes produce a shorter pulse, which I can't reproduce in 1.4.5 so far.