It seems hard for you to change your mind-set, try to think the other way around. Don't hope to find a single reason to explain all of the speed dips, as the causes for different ones may vary. Pay attention only to discs with larger dips for the time being. Test a few more albums from different companies before rejecting the possibility, for gaps may be imposed in different ways. Use pristine discs; do I need to mention that? As I said before, I am not certain about your cases, but since nobody else can offer an explanation, why not consider my suggestion more seriously?