It's just the difference between internal and displayed precision.
sqrt(2) ≈ 1.414213562373095048801
So if you type in sqrt(2) - 1.41421356237, you're just getting the next 10 digits after that.
1.414213562373095048801
- 1.41421356237
_________________________
0.000000000003095048801
yes, exactly :)
that's what i figured out when i first came across this, in school.