Just discussing general theory, I very much understand the difference week to week... I've seen it often. I am very very good at pushing myself in the gym, trust me on that. I like rep ranges to MONITOR progress & decide the level of weight to use, but I've never been the type to stop at 8 reps if I feel I can bust out a few more... the ultimate goal is to have the most efficient time under tension possible (or more simply put, hating your life in the gym).
I know there's a graph out there showing hypertrophy and adaption, its certainly not a straight line graph... more like peaks and valleys that hopefully have a somewhat linear trendline.