Modern Warfare 3 gamemode glitch

Something strange has happened a few times to me. When playing COD Modern Warfare 3 multiplayer (on the PC), I would go to Find Match -> Standard playlists -> Team Deathmatch. Most of the times it would work correctly, but sometimes I would get sent into a game of Domination or Drop Zone. When I am in such match, if I press Esc, the "pause" screen title would say
"Team Deathmatch"

but the description would say
"Capture and defend the flags. First team to 200 wins." (as seen here)

And no, my game is not pirated.

What is this and why is it happening?

Thanks in advance.

(This is one of the reasons for your problem. There might be others.)

This could be a cheater lobby. Those guys can make a real mess out of the game. I strongly advise to not stay in such lobbies as they can as well mess up your personal ranking.
You can identify those lobbies usually by some turning around:

• You get incredibly high score (not always)
• There are more players than allowed in such game modes (this is most common)
• The game mode doesn't match (not always)
• (Someone has 60-0 kills and doesn't even walk around)

Other than that you should try to re-validate your game files. It could as well be a simple error in the localization files.