fixed-width integers have exactly the same semantics as true integers bar overflow, which is a very easy to understand concept.

Floating-point numbers behave like a physicist would do calculations rounding at every step, but doing everything in binary. Or hexadecimal if you prefer, it's equivalent, but still difficult to get your head around to. Then there additionally is + and - 0 and + and - infinity, and several settings for how rounding is done exactly.