What I'm talking about is when they had the picture montage closing credits of Jerry's stand up. Not the closing credits they would sometimes have with Jerry doing stand up, but rather the montages. In seasons 1 and 2 (almost all of the episodes), it's with big letters as you see in the right of the picture. In season 3 they change it up a little (starting with season 2's "The Revenge"), now it's more a mix. Sometimes they would do Jerry doing the stand up as the closing credits, or the same thing as before. But starting with "The Suicide," It seems the closing credit montage starts to change. Now it's smaller in size of text.
No, Seinfeld's closing credits were not inconsistent, because shows are always changing their credits around. Sometimes, a show will change its credits every single time they run them. There's nothing wrong with Seinfeld wanting to change it up a little. Sometimes he did stand up comedy, and sometimes other things, but it was all a fine way to end the show.