Despite your believing that all variables remained the same, something must have been different.... :-?
How do you use your grinder, for example? Do you half fill the bean-hopper before grinding? Only spoon-in the beans needed for each shot? Spoon-feeding the grinder can result in slightly coarser grind size than when the hopper is well stocked.
The tamping force shouldnt be a biggie in the overall scheme of things but certainly dose and distribution is. You may need to examine your technique and the process you use in fine detail, just to make sure.... ;)