On USA Mir regen worked in combat so you didn't use as many pots.
Perhaps you walked during combat? Because regen works not only standing still but also when you walk. Only running deactivates it. But walking on mir is otherwise considered low activity :P' which nobody performs and very few players ever cared for regen anyway because most of the time pots were so cheap and patience low.
Pots taking faster effect (filling up faster where sunpots were at the limit of that - instant recovery) was called (hp/mp) recovery I believe (on the official euro and usa mir) but I think it never worked or perhaps the rate was mistakenly set too low for it to be noticed. o.o
Also not sure how the percentage is calculated. Certainly not from the total like d1craig says above but most likely from the natural regen rate.
This improves with levels and so the advantage of extra regen is felt more at higher levels, it becomes more significant in practical terms.
Didn't natural regen work more often than every 30 secs? Or didn't this time shorten as you went up in levels? Or perhaps mir's 30 secs is shorter than real world time, like 15 seconds given the mir day lasts 12h?