Obtaining this frquency may be impossible if the user tries to use the bits of the counter (as outputs, or to drive other logic). If the user wants to output the bits of the counter they may run out of i/o. The problem arises because in some packages not all the macrocells have i/o. Some of them are buried (they have a (b) in the fitter report). For example, if a 95108pc84-7 is targeted, the largest counter that can implemented at Fcnt is 12 bits. The remaining 4 bits must be routed through other function block(s) to get to available i/o. This will drop the freq from 125 (Fcnt) to 83.3 (Fsystem).
To avoid this problem, target a device/package that has enough i/o per function block for the counter you need.