That is odd.

Generally with an expansion, things get prettier.
And the corresponding requirements for the game go up.
The same system doesn't run as smoothly, or you upgrade a bit.

Haven't heard anything about requirements for Cataclysm.
So don't know if they're the same, or higher or lower.

Lower makes sense in a way, Blizzard is catering to the masses in game instead of the elite 0.02% of the player base.
It could be a decent money maker, to have the game play smoothly on a larger percentage of the computers out there.