They all follow IEEE 754. But that defines the representation of the bits in a floating point number, not what happens when you operate on them. Different implementations do rounding differently, and there are arguments about how to do this correctly. There can be multiple ways to represent the same number even (this is a reason not to ever do a == compare on floating point number that have been operated on). Not only have there been differences between CPU makers, but also between different CPU of the same vendor.

A separate, complicating factor is that floating point math often don't yield the same results with different levels of optimization turned on. The reason to use integer math for games is because you want it to be deterministic so can be an issue.