Missing link/reference on precision in Python - Episode 8: Creating Functions

[jamesacris]

In the Testing & Documenting section of episode 8, there is an allusion to the precision of some data types in Python, when testing the offset_mean function:

The mean of the offset data isn’t quite zero — we’ll explore why not in the challenges — but it’s pretty close.

As far as I can tell, there isn't any other mention of what this sentence is alluding to anywhere in the lesson notes. I could just be missing something, in which case I'd be happy to be pointed in the right direction (though I might suggest adding a link to this section so people aren't left wondering where it is like I was!).

If I'm not missing something and this actually isn't followed up on in the notes, we should consider adding an exercise or add to the notes to explore this further.