Well, this could be a combination of both, although likely more of the second. Looking at your armory, your hit and expertise are OK for a tank... not stellar, but not abysmal either. However, your numbers are better than mine and I don't have many issues holding threat.
Now, on the second point, let me give you a personal example. I have a mage friend who I swear has some mad WoW hax0rz. He was putting out 2600 dps BEFORE the 3.0 patch, and always always always was threat capped. Even after our threat "buffs" he still rides my ass. Most of the time when I'm in heroics or boss fights, I'll be generating 2.5-3k tps on bosses and by the end of the fight be miles ahead of the dps. He, however, will always find a way to keep up or at least be closer than anyone else.
Long story short, if you're running with a bunch of pro dps, they will definitely make your TPS an issue, especially if you're not doing a good job of it.