Probably you have a DualCore Processor, or a Hyperthreading supporting Processor, in either way, that will cause your F@H To use only 50% of the total CPu Power that is avalible to you, if your CPU is just a dual-core, it should be theoreticly as much as 100% if you compare the processing speed to a single core [50% of two cores = 100% of a single core]
if its a single core CPU with hyperthreading, then sadly you're limited to half the power that your CPU can utilize for F@H, what you can do is run two clients [download an x2 agent,there are a couple on the net].
Hope this explains some stuff,
-Jo