Most variety of diameters only in .22 - why is that? And why undersized?

You are spot on! Seems like .22 barrel manufacturers have more different bore sizes than in .25 for some odd reason. My RAW's LW non choked poly barrel only shoots .2175 diameter NSA slugs accurately and my Taipan V2's CZUB barrel prefers.218 NSA slugs! My friends M3 barrels likes .2165...but most .25 have only a couple sizes available that shoot accurately from most barrels in .25 it seems.

Chokes and smaller diameters to resize bring their own issues where there's less issue in 25. Caliber restrictions in countries like south Africa where the biggest salesmen connected to manufacturers live.... Maybe. 22 has been mainstream longer than 25 in the modern market and maybe was less standardized but maintains a higher demand. I'm only guessing here.

Different countries, different sizes..

It’s equally valid to question whether 22 is the most difficult caliber to find the best slug since all of those diameters exist for a reason.

Mlydon said:

Why is it that in “.22” cal, most major manufacturers offer .217 and .218 (so they’re undersized quite a bit, and Nielsen goes all the way from .216 up to .219, yet in .25 it’s mainly .25 only, with a few .249 (under), or a big jump to .253 (over). In .30, that’s it, with patriot and Nielsen offering .301 and .303 as well - no under sizing…

Matt

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.22 airgun barrels are not .22 diameter as a PB barrel is. They are usually around .217/.218 and that's typically unchocked. Some chokes can push them down to .212 from what I've seen even. It's confusing for sure. I'm not sure why we call them .22 when .177 aren't called .18. But anyways yeah the slugs aren't undersized that's just what the barrels sizes are.