Why can't I shrink my database any smaller than xxx Mb?

A. The probable reason for this is that you're still on 6.5 and below. With this setup you can only shrink a database back to the last portion of log. So if you defined a database as 400Mb data, 100Mb log you can never shrink it. 

If you defined it as 400Mb data, 100Mb log and then expanded by 500Mb data you could then shrink down any/all of this last 500Mb assuming it wasn't being used.