The simplest possibility is that all of the items that can stock in any section of the treasure MP are in the pool and one is simply chosen at random.
But I'd think if that were all, it would be an apparel item far more often than the rest of them put together (though I haven't run numbers on that) and that doesn't seem to be the case.
Also, limited-time items seem to come up more frequently than they would if it were pure RNG (Spring's Breath has been, what, at least three times now; I know I've seen a couple of fest skins too.)
One roll for which section followed by a second roll of the items within the section would solve the first issue and would explain the fest skins as well - but not Spring's Breath. So maybe that with a slight weighting on limited-time things as well (like happens with Swipp)?
Thoughts on flash sale selection mechanics?
But I'd think if that were all, it would be an apparel item far more often than the rest of them put together (though I haven't run numbers on that) and that doesn't seem to be the case.
Also, limited-time items seem to come up more frequently than they would if it were pure RNG (Spring's Breath has been, what, at least three times now; I know I've seen a couple of fest skins too.)
One roll for which section followed by a second roll of the items within the section would solve the first issue and would explain the fest skins as well - but not Spring's Breath. So maybe that with a slight weighting on limited-time things as well (like happens with Swipp)?