Nate is on holidays right now so I won't give you the best answer but I think it depends on two factors. The biological system in question (simulation box dimensions / number of atoms to simulate) and also if they use some CPU functions and how often they use them.
Before some months Noelia was working with some CPU bound functions so this caused her a slowdown, but at this point I think she doesn't use them anymore (I think we implemented them on GPU's) so it probably depends on the biological system.

As on credits, I don't personally know the formula with which they are calculated, but they cannot be done very exactly (apparently). So there will always be some (at least minor) disparity there.