Hi hsnumr
During my workings about the microstrip patch antennas, I observed that, position of the inset feed changes the antenna return loss value and also the antenna bandwidth not the resonant frequency.
For example, let's say return loss (S parameter ) was found -8db when inset feed position was chosen wrongly and it was found -15db when inset feet position was chosen properly but those are both in the same resonant frequency such as 2.45Ghz.
My suggestion is, You should control the length and width of your patch (especially length ) to obtain true resonant frequency value.
Regards eldamar..