See, that bit contradicts...That bit.
The hard drive is one file. OS X will treat it as a single contiguous file yes, but inside the file is a working model, if you will, of a HD. This virtual HD will become fragmented as windows slaps data anywhere it wants, just like a real computer. OS X has no control over it.
My point is, who cares if it's fragmented. It's treacle slow anyway.