I'm sorry, was my original explanation not clear enough? I'll break it down for you.
Card A averages 100FPS
Card B averages 50 FPS
From the perspective of Card A, Card B is half (50%) as fast. From the perspective of Card B, Card A is twice (200%) as fast. It's always relative to the starting...
Registering solely to point out your stupidity. The 3870 is 56% when the base(100%) is the 4870. If the 3870 was the base, the 4870 would be nearly twice as fast. Guy above me made the same mistake.
Apparently posting on a tech site doesn't require that you pass third grade math.