Which means they are rough ballpark figures. Trap speed is an average over the last 66', not the actual speed you were travelling crossing the 1/4 mile line. So trap speeds tend to read a little high on in-car devices. In addition, wheel spin throws everything off.
It's data. In the absence of anything else, it's useful. But it will usually think your car is faster than it really is, compared to real dragstrip times. On the other hand, track prep and weather conditions influence your drag strip times quite a bit, so timeslips aren't really comparable either.