https://community.fantasyflightgames.com/topic/296488-i-tested-my-dice/
Once of the mad lads over on the X-Wing minatures forum did an extensive test on his X-Wing dice, and found that manufacturing flaws created a bias in the dice rolls.
Now, at the price point of mass market gaming dice, there was likely to be some bias, but he found up to a 5% bias on one of the dice! This is more than the influence of some game mechanics!
I'm sure this kind of bias exists in all mass market gaming dice, but I wasn't sure exactly how much to expect. I thought some might find this interesting.
Also this:
https://www.awesomedice.com/blog/353/d20-dice-randomness-test-chessex-vs-gamescience/
I remember a similar issue with GW's dice for Warhammer many years ago. Makes sense mass produced cheap dice would have bias issues.
Pretty sure the GW ones still have this issue.
...or my friend just rolls like shit all the time. Constanty. With re-rolls.