AMD's construction core CPUs (Bulldozer, Piledriver, Steamroller, Excavator) share resources, so they're arguably somewhere in between a 2c/4t CPU and a true quad core. When you load up all 4 cores, there's a penalty to performance so it's not quite as fast as if the cores didn't have to share. The reason it shows up as 2c/4t in Windows task manager is that the cores are in "modules", or pairs of two, and Windows needs to be aware to load up only one core per module before loading the second - e.g. load up "cores" in the order of 1, 3, 2, 4, because 1 and 2, and 3 and 4 share resources, in the same way that on an Intel CPU with hyperthreading, you want to fill all of the cores with only one thread before adding a second thread to any core.