…In other words, a Floating Point Error, or so my web research says.

Apparently, in all coding languages (not just Flash), a decimal does not have an exact representation in binary. So, for example, 5.100000000000000001 is seen as close enough to 5.1. This is fine when doing other things, such as moving a Movie Clip to an x position, but if you are coding something such as a calculator when you need to display the exact number, it can be frustrating.

I found a method online that corrects this by comparing a few numbers, and estimating. Seems to do the trick in my case.