It's usually referred as "CPU scaling".
To put simply, with the few exception such graphic demanding games like Crysis 3, crossfire 7970 should be able to do minimum frame rate above 100~120fps without issue, however for CPU, it is not uncommon for an overclock i5 to have a minimum frame rate around only 70-80fps. The actual frame rate you get in games will always be limited the lower of the two (CPU frame rate vs graphic card frame rate), so despite crossfire 7970s themselves may have no problem holding 100fps+, if the CPU minimum frame rate is only 70fps, the minimum frame rate you get in game would still be 70fps.
Would that explain why my GPU usage does not go as high as I would like as the CPU is holding back, or could the GPU's still be at 90-100% and still be held back by the CPU?
Tom