In a previous blog written by Brad Case (Sept 24, 2019), he explained how to perform a repeatability test and why it is good idea to do so.
Here are few other items that have contributed to repeatability issues.
1) The top rails were not sufficiently clean prior to taking readings. Paint was not removed, and rails cleaned with Scotch-Brite pads at the measurement locations.
2) Threaded holes at the measurement locations were not stoned to remove high spots.
3) The receiver was not placed at the same location for each reading.
4) Components were loose on the receiver or the laser transmitter or both.
5) Receiver lens was dirty.
6) Transmitter lens was dirty.
7) Receiver was out of calibration.
Many times, it is little things that can cause big headaches.