There is actually no NDK that matches the complete current NIP list and is 💯 backwards compatible. They all pick and choose what to focus on and which changes to support.

And, increasingly, new Nostr specs aren't going to be NIPs. They're going to land in the wiki event register or be product specs in those individual product repos. Or just be open-source prototype implementations. We will be implementing a selection of those, as well.

So, yes, different NDKs are different.

https://wikifreedia.xyz/nip-event-register/laeserin@getalby.com