Actually it's the dragstrip that is way off.
Tracks average your last 60 ft. rather than measure your exact mph as you cross the finish line. So at the track your trap speed is actually lower than your finish line mph.
If you don't believe me, take a look at your speedo the next time you are crossing the finish line.
Unit5302, I have tested my GTECH at the track and found that the GTECH will show almost a tenth of a secong slower than the track. This is because at the track the timer starts when you break the staging beams, not when the car starts to move.
At the track you can pick up a tick if you stage very shallow. Just so the 2nd pre-stage light comes on.
At the track you could very well see a 13.9 @ 100-101.