A. Currently XAMPP for Mac is on a different version (1.7.3) than XAMPP for Windows (1.7.7). You should look on the XAMPP Mac Forum.
B. Generally, the different OS versions of XAMPP with the same release will contain the same versions of the major components (the AMPP) Apache, MySQL, PHP and Perl.
C. All the 'other stuff' is often platform specific - for example - FileZilla server for Win, ProFTPd for Mac, Linux, Solaris. Webalizer is included in most versions, but the Linux, Mac and Solaris versions work differently than the Windows version. Mail is different on platforms, ditto backups and file permissions. The Control Panels are all different. One thing is the same - XAMPP is OS agnostic on all platforms, meaning you can install it, and remove it without any chance of breaking or needing a repair to your OS.
Despite what any convergence gurus may say - there's still a lot of methodologies, required knowledge and experience that are closely tied to both the OS and processor architectures.