Quartus9.0 Vs Quartus11.0,some question about the PLL

hello,

I got a project needed a pLL with the 12.8MHz input clock and the 24.576MHz output clock .And I choose ep3c16240c8 as my device.The software is Quartus11.0.

then,the pll setting shows just like followed picture.

http://www.alteraforum.com/forum/attachment.php?attachmentid=12214&stc=1

Actual settings shows that the true clock is 24.58814

then i got the same setting and device in Quartus9.0 .

http://www.alteraforum.com/forum/attachment.php?attachmentid=12215&stc=1

Actual settings shows that the true clock is 24.576

so ,why?and ,could ep3c16240c8's PLL set the magnification as 48/25?

thank you