Pontoon Basic Strategy: 2 card 9 vs. 3 card 9

[TGoldman]

Could someone please explain the reasoning behind the basic strategy for Pontoon that involves hitting on a 2-card 9 vs. doubling on a 3-card 9?

If we're doubling hoping to eventually make a 5-card trick, why not wait to double until after we have more information knowing our 4th card? The other reason would be to double on the strength of the 9, but if that's true why not do it for all 9's and not just 3 or 4-card nines? [img]/images/graemlins/confused.gif[/img] Any ideas?