As I'm pretty sure this is totally incorrect, I would expand on my former statement in saying that similarly to Genius in iTunes, iTunes Match works by scanning your ID3 tags. It is also more dynamic like Genius in that as it matches more songs, it will become more accurate in finding matches for the songs in your library.
I'm pretty sure it matches by scanning the audio, like Shazam and Soundhound.
The following isn't meant to be a shameless plug:
I'm a musician and I have two rock albums available in the iTunes Music Store. In my iTunes library, I have my songs as well as their instrumental versions. Then I signed up for iTunes Match...
Naturally, it matched the album versions, but for a couple of the instrumental versions, it matched them to the full album versions and the rest were uploaded. The ID3 tags and artwork are definitely not the same.
I'm hoping there's a way to 'force upload' to iCloud.
The album is called King Friday, by the way.
http://itunes.apple.com/us/artist/richy923/id301066330
...okay maybe a bit of a shameless plug