I've completed the Legendary campaigns of Halo 2, Halo 3, and Halo ODST. I've slogged through the Veteran campaigns of Call of Duty 2 and Call of Duty 4, including rescuing a hostage from an airplane full of terrorists in less than 60 seconds. I've even managed to get through a complete episode of Left 4 Dead 2 on... well, Advanced, not Expert.
And yet, when I attempt to play on Xbox Live, I might as well bring a knife to a gunfight.
I don't know what it is, whether it's incompetent enemy AI or something in me that just chokes when I go against human players. All I know is, this music video sums it up nicely. :)