I think they're buying based on how much available U.S. aid money they have at their disposal. Sartaj Aziz said that Pakistan will fund the purchase of the 8 F-16s through "saved" FMF funds. I imagine the White House was looking to ease Pakistan on that front with additional FMF, so that the PAF could buy the total 18 it had originally wanted, but as expected, that new release didn't go through (yet).
That said, I've long expected the PAF to keep buying new Block-52+ on an incremental and gradual basis. They'll order 8 this year, perhaps another 10 next year, and so on. Before the earthquake in 2005, the PAF had intended to acquire a minimum of 55 new Block-52+, I think they're still aiming for that. It would be enough new airframes to warrant an F-16V-type upgrade 10 years from now.