We’ve seen poor quality (e.g. aliexpress) belts vary in pitch by a percent or more. Belts from V1E have been tested more and observed to be more accurate and precise, without having to pay premium prices for premium branded belts.
Depending on where your Belts were sourced from, check out…
The accuracy and precision behavior you’re seeing for repeated tests can be helpful with identifying correct thing(s) to focus on fixing/tuning.