When you promote your crew, they undergo hours of ardurous training. As a
result, they get tougher (i.e. gain max hp), but are tired (gain hp, but not
as much they gained max hp). This tends to make your crew look wounded, until
they regen up to their new max.

This rationale makes perfect sense to me, until you hit golems. They're lumps
of material that've been animated by magic. So, from now on, when you promote
a golem, its hp and max hp go up bythe same amounts.

Melemkor