Why would we assume non-interstellar comets are always the same as interstellar comets? Conditions obviously are a little different when something is ejected from a system and then spends millions of years in interstellar space.
> Borisov had the same characteristics.
We have a sample size of three thus far. Making conclusions right now is like saying all extrasolar planets are large gas giants because the first three were.
We're still figuring out what our comets are like, let alone unusual ones spending a few million years in interstellar space. New types of comets(ish) bodies discovered in the 2000s:
https://en.wikipedia.org/wiki/Active_asteroid
https://en.wikipedia.org/wiki/Manx_comet
We've spotted ~5k out of an estimated trillion. Each one we've sent a probe to has brought surprises. The Oort cloud remains theoretical at this time, and the first Kuiper belt object other than Pluto/Charon was found in 1992. It would be deeply silly to think we know everything about our local comets, let alone unusual ones from elsewhere.