"Gamescience dice are more consistent than the X-Wing dice", with some analysis regarding the flashing from the mold suggesting that sanding it smooth will increase the consistency.
They _manually_ tested Chessex and Gamescience dice by rolling them thousands of time. They found both brands' dice to be off. Gamescience dice had a big dip on the number 14 face, which is the face that has the bit of flash where the die is cut off the sprue by hand.
I suspect it's just easier to make a fair d8 than a fair d20.
That probably goes for other die sizes also. I was looking for a d14, d16 and d18 and while I found the first two from Gamescience, I could find a d18 only from one brand, other than Gamescience, so I'm guessing that Gamescience decided they simply couldn't make a fair d18 work. I mean, why skip d18 when you have d14, d16 and d20?
"Gamescience dice are more consistent than the X-Wing dice", with some analysis regarding the flashing from the mold suggesting that sanding it smooth will increase the consistency.