D20's aren't balanced because there are more 2 digit numbers than single digit numbers. Since the digits are carved into the face of the die, that's a little less weight on the side that favors two digits. Originally, you were supposed to take a crayon and fill in those numbers so they would be more visible, but it had the added benefit of counteracting the imbalance by adding the weight of the crayon wax back to the carved out digit locations. Unfortunately, people were too lazy plus the added benefit of illegible dice being much more cheatable, thus the technique never caught on. Now, D20's use a die stain that is nearly weightless which aggravates the imbalance.