9,9 VS. 7

I am very loyal and have much faith when it comes to basic strategy. However, I've always been curious about how the gears to everything works...

When you've got a pair of 9's (Split to 2-9 accept for 7...that's the multi-deck strategy, at least) against a dealer's 7 why don't you split? When I get that hand, I stand of course because basic strategy says so...but this is one of the only plays that's made me think WHY DAMNIT?! =)

You split them if the dealer shows an 8 or 9 or 6...so why not a 7? I must be thinking too hard. Hmmm...

As always, replies are greatly appreciated as there is much wisdom to be soaked in here.

-Pard Pounter