Whats up with Mumbaikar calling 11th and 12th college?

Wait, it isn't called college everywhere else??

It's just a function of having pervasive primary and secondary education available across the state for decades. Most of the older schools in Maharashtra only have classes till 10th. It's a hangover from the past when Board exams were in the 11th and then a year of Pre-University after which people used to decide what Bachelors course to get enrolled into. Elegant system if you ask me, helped people make informed choices. When the 11th boards got split into 10th & 12th - the school's stopped at 10th and the colleges took over and offered the 12th classes. Hence for the past 50 yrs most colleges start at 11th and go on to Post Grad. Thats why 11/12 are called Junior College.

It is one of the funnier variations in India 😂

I know folks who call degree 1st year as 13th, 2nd as 14th and so on

I'm from Karnataka and it's called PU College here as well while Mumbai calls it junior college.