That is well with the experimental error.
The difference in draw length and loose can easily cause that much discrepancy...
There is also the possibility of false triggering caused by arrow flex etc... e.g say the arrow is flexing and instead of the point triggering the first gate, it is just outside the sweet spot and the shaft 2" behind the point flexes into the sweet spot and triggers it.... then if the point itself triggers the second gate it will give a high reading ... as it's effectively been give 2" head start!
Poor lighting or strobing from the light source can also cause problems (florescents or LEDs can do that.. incandescents are ok).
Del