The manticore's maximal number of hit points, for some purposes, is 89, as you would expect. I think that if the 403-hit-point manticore were damaged in combat until it had only ten hit points left, but then healed because somebody cast Regeneration on it (still in the same battle), then it would start to heal, at something like five hit points per turn (5% of 89, not 5% of 403!), until it reached 89. And then it would stop healing, and things wouldn't look strange anymore. For that purpose, the maximum of 89 is a hard barrier, as you would expect.
But for some other purposes, the "maximum" that the unit's profile seems to advertise is not really a maximum. For instance, a unit can gain hit points beyond its seeming maximum by casting Drain Life during battle. At the start of the next turn, it might have "23 out of 15" hit points remaining.
Similarly, a prophet or god that moves around can have its seeming maximum dropped below the hit point count it had before moving, as you've seen. But the hit point count doesn't itself drop in the same way. Here's how hit point counts change between battles, for creatures that are capable of healing (that is, creatures that aren't diseased or skeletons or whatever):
If the unit in question has less hit points than it would normally have when fully healed, then it gains back all its hit points.
If the unit has more hit points than it would have when fully healed, then it loses something like half of the "extra" hit points... I forget exactly how many. So, a prophet moving into increasingly intense enemy dominion will lose a chunk of health every turn, but will always have more health than it seemingly should, because its "normal" hit point level keeps dropping, and its actual hit point count lags behind!
Well, I like explaining things. I think this is all correct, anyway....
