Not incorrect measurements of the earth's circumference, rather he used incorrect maps that showed Asia as being like 4 times bigger than it actually is, with the Polynesian islands and Japan being a continent sized chain, each at minimum the size of Sardinia

The reason people think he was using an incorrect circumference is because one of his journal entries can be read to either say he was close to mount purgatory, which would have been super far from where he'd have been, or that he believed he was heading in that direction, which to be fair he actually would have been had the mountain existed.

I forget who but someone put it like this, if you asked Columbus for what continent he was on, he'd have been wrong, but if you asked for his coordinates on the earth's surface, he'd have actually been pretty close to accurate.

He knew "where" he was, he just didn't know where that where was.