Well,
The simplest explanation is that MacRuby is a Ruby 1.9 VM. In earlier versions, it was a modified version of YARV (ruby version 1.9), which, instead of using custom types for things like ruby strings, hashes, etc., used equivalents found in apple foundation classes such as NString. With the advent of version 0.5, a whole new LLVM-based virtual machine was created, again compatible with ruby 1.9, which is based on the base classes of apples.
1.9 VM. - api-, , , MacRuby (, HotCocoa).