It's not that simple. +400 simply means you don't mind playing opponents in that rating range - you're not guaranteed to get opponents rated 400 more than you are.
Also, you're not the only one looking for a match so the server can't be picky; it selects the closest match it can find.
I tried to experience the selecting rating range of my opponents. If I try to select My Rating -> + infinite i receive players of high ratings but not very much, and if I select range - infinite -> +25 Rating I receive players real close to my rating. Why is working like that? If I select +400 give me +400, if I select -400 give me -400.