A wooden die that has carved pips with no active attempt to prevent bias (aka, a weird weight in the middle, etc), will exhibit a bias from the fact some faces have more pips (which are pits in the wood) than others. This means some sides are more likely to be pulled face down, and end up face down.
That is why in vegas you use those certain red dice at a craps table http://en.wikipedia.org/wiki/Dice#Precision_dice
I play catan with craps dice.