The current wording is, IMO, better than the suggested wording, especially if you don't want to be "teaching stuff".  The current wording is a specification of the method's behavior.  I really don't know what you could replace "approximate" with that would improve it without having to get into a description of the behavior of a threaded program.

It seems like you are wanting us to document that the function will return an accurate size if the program is single threaded, but I don't think we want to do that, because that is not part of the specification of the method.