I would guess that either your form is inconsistent, meaning varying a bit shot to shot, or your anchor isn't that consistent.
I've never repeated draw length measurements but when I shoot over a chronograph with a new bow/arrow combination I shoot 12 arrows and throw out the high and low. I enter the rest into a spread sheet to determine the average arrow speed. Along with that I have my spreadsheet calculate the standard deviation. The lower the number the more consistent the arrow speeds are and I think the more consistent my draw length and form are. (You are not the only one that overly analyzes things.)
The standard deviations I have gotten with my three bows are:
35# Martin Howatt Hunter - 1.1
55# Martin Hatfield - 2.2
Darton Compound - 1.8
These are pretty low so I'd say that I'm pretty consistent. Now if I could just be as consistent on target. :mad: