It's actually not an error. It's simply math rounding.
Take 3.3+3.3+3.3.
Round each individual part, you get 3+3+3=9.
Round just the answer and you get 3.3+3.3+3.3=9.9 rounds to 10.
It's not an error, mathematics was made that way. Blame the Egyptians, Babylonians, and Greeks. <_<