Hi @helen_y, not a naive question at all, and certainly one shared by many. I’ll do my best to shed a little light…
For the examples you gave (and in most examples of name changes these days), it comes down to the choice of a taxonomist whether to lump a group of species into one large genus, or to split them into smaller genera. There are many scientific factors that go into such decisions, but in the end they are arbitrary and subject to differing opinions.
The Code of Nomenclature maintained by Botanical Congresses provides the various ranks (like genus and species) available for taxonomists to use, and the rules for determining the one correct name for a taxon with a particular type, rank, and circumscription. It also provides for “conservation” of younger or otherwise incorrect names over older correct names, when doing so would avoid undue nomenclatural instability.
Beyond that, however, the Code does not tell a taxonomist how to arrange species into Genera, Families, or any other rank categories. That is up to their interpretation of the available scientific (hopefully) data on those species, which these days generally means their evolutionary history as inferred from DNA studies.
Once evolutionary lineages have been settled, however, it is still arbitrary as to whether to put several lineages of related species into a single large genus, or divide them into smaller genera. A taxonomist who wants their classification to have the best chance of gaining universal acceptance will want to put a large dose of common sense into their decision.
Choice of genera is particularly important, since the genus name is part of the species name, and so the most “visible” part of the classification. Are my genera easily recognized and distinguished by ordinary practical characteristics? Do they make evolutionary sense? Etc.
Sometimes there are pretty clear answers to those questions, and other times it’s a coin toss. And that’s where names can continue changing back and forth between different genera (like your examples), depending on the whims of the day. In general, a classification that strikes the most practical middle ground between the extremes of lumping and splitting will have the best chances of long-term acceptance. But it can take a lot of back-and-forth to find that middle ground, and then new scientific discoveries can sometimes throw a big wrench into the works and start the process over again.
Wish I could tell you that someday names will become completely stable, but that would be the day that scientific discovery ceases, and/or nomenclature becomes a dictatorship. Definitely hoping neither of those come to pass.
As for choice of POWO over APC, iNaturalist had to decide on a single worldwide taxonomic framework for each major group of organisms, to maintain as much stability and consistency as possible within iNaturalist. But it does allow for “deviations” from that framework when there is some reasonable consensus in the iNat community. So if you find yourself really disagreeing with the POWO classification for a particular taxon, you can always flag it for curation, and explain in comments why you think a different classification should be followed on iNaturalist. Many such deviations have been implemented over the years, when there was agreement to do so.
Hope this helps, and I’m sure others will have different perspectives to add also.