The Head Hunter

Obviously movies can do whatever they want, but I'd say the difference is that he was just smearing a handful on himself at various times. That was enough to heal him from his current wounds, and perhaps might have had some lingering effect for a day or two that we never saw, but it was pretty obvious that he didn't keep regenerating because every time he came back fucked up he had to apply more. If he had accumulated enough to regenerate like the monster, he'd be healed by the time he got back after every fight. It obviously wore off at some point.

The creature got a massive dose, all at once. Like, tens to maybe hundreds of times what the hunter used every so often. And, it was only regenerating the head, not various wounds all over his body. We never really were told time frames, but clearly the implication was that the bounties were coming faster and faster, and that's why he started to get sloppy with stuff like not locking the black goop up every time and not fixing the shutter. If he'd had even a single day of rest before that last hunt, he likely would have tidied all that up and none of this would have happened. So except for the end, his applications likely had plenty of time to wear off even if the effects were cumulative.

Anyway, point being, TL;DR I think the difference is the massive single dose the monster got. It probably would have eventually stopped regenerating, but not in the day or two they fought after the first kill.