Why does Apple Activity Monitor report that my Mac with a dual-core Intel i5 Ivy Bridge CPU has 4 cores?
This is a technology called Hyperthreading those i5 chips support.

It means two threads can run simultaneously on each core resulting in two additional virtual cores. OS X's Activity Monitor only shows virtual cores, not physical cores. Likewise, a quad-core chip has eight virtual cores and that's what's presented in Activity Monitor.

To sum it up:


1 CPU
2 physical cores
4 virtual cores (2 per physical core)