In addition to the "International Settings" System Preference portion of Mac OS X (which is software-based support for internationalization), Apple also offers hardware-based support for internationalization in the form of different keyboards for different languages.
In the US, we have the option of selecting a US English keyboard, or a Spanish keyboard (for those close to Mexico, I assume?). The Spanish keyboard differs in that certain accent and language characters (what are those called, again?) are present on the keyboard and issue a different keyboard "code" to the operating system when pressed.
It stands to reason that UK English keyboards would have a pound sign instead of a dollar sign, among other subtle differences, and would be "similarly different" in other countries as well.
You can easily change the software-based international settings. You can't as easily change the hardware differences.
Since a lot of users are based out of the UK and a lot are US based, it stands to reason that one person's "shift-3" or "option-3" produces a different character than someone else's. This is partly due to the software- and partly due to the hardware-based internationalization.
jbarley, you're talking software. fryke's talking hardware.