Workers, Pages Functions & 10ms CPU Time

Hi,
Studying what could fit in the well-known "10ms CPU Time", I was surprised by some "discrepancies":

the same code in a worker is going to be (statistically) slightly faster than in a page-function,
that same function - if the only one - is going to be (statistically) slightly faster than grouped with others even non used.

Of course, it's never much, but enough to make it jump over the 10ms!
Is that logical, even expected? Anyone has done some real extensive tests?