Yeah, i second what Southpaw said.
Time and again i'll hit local tournies and play against people i've played with online. Online, i've found i get beaten pretty regularly; but once we get on LAN, the roles are reversed. LAN is a completely different event, and even if everybody had equally efficient connections for XBL, simply because you can't shoot across map forces people to play differently on XBL than they may on LAN.
As a great example, FBI Agency has already beaten FB and Carbon in the online tournament. That happened in a single online tournament.
Looking at LAN then, they've been to multiple events, and haven't been able to repeat such results on the LAN.