Hi, I highly recommend my old posts on the subject:

In Columbus' time, what were the competing theories about the circumference of the Earth?

Circumference and size of Earth in Columbus time

In them I demonstrate, among other things, that the idea that other people had a correct knowledge of Earth's size is just not true, and Columbus' peers ideas of circumference (and there were multiple numbers floating around) were also wrong. It's just that Columbus' number happens to be less correct, further increased by his arbitrary increase of size of Eurasia landmass (it is covered in my posts if you want to read more)

It wasn't the math that was the problem, it was the unreliable methods of measuring large distances, as well as rudimentary methods of latitude taking on sea, which introduced errors that were hard to compensate for. And none of the sailors at the time just couldn't get it completely right. The incorrect size of Earth and length of one degree used by Portuguese and Spanish respectively continued to be used for quite some time after Columbus (and that's the values Columbus rejected!) so the problem isn't really only in Columbus but the tools and method. Not to mention Columbus proposal came in time (1480s) when latitude measuring for navigation was really a new thing (in Europe at least).

As for your question if Columbus knew the truth but lied anyway, there really isn't any evidence for it. It's in his personal notes (marginalia on some books) where we find references of his strong belief in the incorrect value of size of Earth, and his claim he got the values by measuring and from other evidence. And there is no reason for anybody to suspect that Columbus would lie in his personal notes whom nobody will see, is there? So on the one hand he have his public and private writings that match and give incorrect values, and absolutely no other indications or anything to suggest he could have thought otherwise