I've played the original Fallouts multiple times, so I get the concept. But I don't understand Underrail's mechanics.
My current character is a stealth-psi build. When he initiates combat sometimes he gets 3 attacks, but sometimes he only gets 2 or 1 (1.5, technically, since he has enough AP for Neural Overload). He's got Tranquility, so that usually explains the 3 vs 2 (whether he's at full health), but there are some hidden variables that I don't get.
For example, when doing the cave hopper quest I'll be at 100% health, but often get stuck having to skip many turns in combat mode, and the AP/movement meter sometimes varies. Or with the evil sentry bots and cameras at the GMS compound: I was just playing through, reloading and retrying, and I get different AP and movement points in different situations at 100% health.
I know that certain attacks and traps effect AP and/or movement, but there has to be more going on.