That's always been an interesting question to me. Yes, the capacity of the MTS Centre is just over 15k. So sold out every night, it would still land them in the bottom third of average attendance. But my unscientific opinion is that a lot of arenas and stadiums are actually a bit to big: perpetual sellouts and a tough ticket are good for demand, good PR, and a good experience for the fans in attendance--making them want to go back or consider season tickets. A few years ago when the Habs were missing the playoffs, the arena was not always selling out--and even though the average attendance was still higher than the capacity in almost every other NHL arena, it was reported as a negative and fueled the (clearly ridiculous) notion that Toronto was the only untouchable Canadian franchise at the time.
That kind of thing is poisonous, and I think a sold-out 15k in Winnipeg is better than averaging 13k in Atlanta, for example, even if the potential for a bigger crowd is present for the Thrashers. I'm sure a "ceiling" is viewed much more negatively by an investor and the league, but I also think they are making projections based on crowds that aren't there. I think if the casual fan can't get an easy ticket to the game, they are MORE likely to watch it on TV and increase the franchise's ratings. If they can go anytime they want, neither the live or televised event seems like a must-watch.
Anyway, I grew up in Phoenix so I'm rooting for the Thrashers to move instead of the Coyotes...