Just finishing up an especially difficult upgrade, and hit a glitch on what should have been the easy part. Went from 1.1.12 -> 1.1.21 -> 2.0.1 -> 2.0.15. When I uploaded 2.0.15 upgrade package and ran it, got the outdated files error. Uploaded 2.0.15 install package, ran upgrade.php, upgrade successful. Everything else was the same for both packages. Have done a lot of upgrades, don't think I ever saw this before. (http://www.thekrashsite.com/pics/idk.gif)
Could be the language files and the info it's checking for don't match, not a big issue though.
Don't know what caused it, was able to replicate the problem on test install. Did another clean install, d/l'd the current 2.0.15 upgrade package, worked fine. (http://www.thekrashsite.com/pics/idk.gif)
The upgrade package should include up to date english language, but I know that message can come up with any other language used - including english-utf8, if you do not also update the language files or switch to english. Don't know what else might trigger it though, apart from the original upload of the upgrade files faiiling partially for some reason perhaps.