I think a lot of that is due to how certain cells are spec'ed. If their CDR rating is set at close to where the cells practically catch fire then they will have a lousy voltage under load due to being pushed too hard for their internal resistance value. But if the manufacturer sets a conservative CDR then the cells are, comparatively, loaded less for their internal resistance value and the voltage under load will be higher. Temperature measurements of two cells at the same discharge current level can also tell you a lot.