HEAD is "the thing we're editing now" but that's not terribly useful when rebasing since you're repeatedly editing a fake history.
The meaning of "ours" and "theirs" is always the same, but the "base" of the operation is reversed compared to what you might be used to during merge.
Rebasing can be confusing and hard and messy, but once I learned that rule and took the time to internalize it, I at least never got confused on this particular detail again.
> fake history
That's the thing, it's not actually fake history. Git really is doing the things it looks like it's doing during a rebase. That's why you can do all kinds of weird tricks like stopping in the middle to reset back a commit in order to make a new intervening commit. The reason you can abort at any time with (almost) no risk is because the old history is still hanging around in the database and won't be removed until GC runs, usually long after the rebase is settled.