When I say "appeared at 25%" I mean that I'd previously deleted (in battle) their entire army, and that one newly-raised brigade (likely from soldier POPs) started occupying territory when my occupation was at 25%, and reached 70% occupation when I was at 70%.
And yeah, 180 days does seem pretty fast... maybe it was 280? Maybe I'm confusing some things, but it definitely happened in a way similar to that.
I think the set of circumstances that would make the most sense is something from this list:
1) A unit gets a bonus to occupation speed when occupying owned and core territory (each bonus is applies separately).
2) A unit that is unable to reinforce at max speed (i.e. lack of soldier POPs) either A) Imparts a penalty to the army (so a 7-brigade army with, say, 5 brigades lacking soldier POPs to reinforce would occupy at 9/14 speed or something) or B) Occupies territory at a speed proportional to how many soldier POPs are left (so a brigade with 500 POPs would occupy at 1/2 speed), or a mixture thereof.
3) A leader's attack or defence skill is somehow involved (The Argentinians at this point had a 6-defence general while I was still running around with Bolivia's starting +1/+1).
4) Small armies and/or nations with small armies get a boost to occupation speed.
5) War Exhaustion affects occupation speed (Wouldn't explain this particular case from Peru's side but I was running around at ~30-40% by this point).
6) Mobilised units occupy slower for some reason (would coincide with reason #2).
7) Military funding actually does affect occupation speed (maybe ramping up to 2x speed at 100% from >90% or something because the AI was definitely at 100% funding).
8) There's something glaringly obvious I'm missing.