Ever since OS X 10.6, the OS has been calculating file sizes to coincide with how HDD makers have been doing it. Bytewise it's the same but it does cause some confusion when an OS X person says one size in MB or GB and a Windows person's filesize says something different.
I also talk in the Nintendont thread and it's come up a few times where a Windows person said the ISO was corrupt because the OS X person said it was 1.46GB when in Windows it says 1.35GB. I have to intervene and tell the Windows person that OS X calculates file sizes differently and that 1.46GB in OS X = 1.35GB in Windows. What a pain when a Windows person isn't aware of the differences in OS X. Apple should have never changed it.
Any particular reason you are still using an ancient version of rxTools?