Oscar and Elmo are good examples. Like Scott said, they were not Sesame Street Series 3 or 4 , they were just "Sesame Street Oscar" or "Sesame Street Elmo", but they were handled just like the previously two SS lines.
I think it's the same thing for Fett. It got a standard Kubrick Series number (No.246), but it isn't called "Star Wars Series 11" because it's not technically SW Series 11 - it's just "Boba Fett Collection".
It may have been less confusing for folks if they had called it SW Series 11, but for whatever reason they chose to keep it separate as it's own thing.