EXEC = number of retired instructions / number of nominal CPU cycles
the number of nominal CPU cycles during a time period is defined by the nominal frequency on the processor, which is advertized in the product. For example if you have an Intel Core i5 3.3 GHz processor, then during a second you have 3.300.000.000 nominal cycles.
However, the processor can be idle most of the time during this second such that the clock signal is not running (in apower saving state). Therefore to adjust to this fact that processor was in power saving state it makes sense to have a differentmetric that counts only time (cycles) with active clock signal. Thus the IPC metric divides the number of retired instructions by the number of active clockcycles.