Your MacBook Air has 2 independent processor cores.
When Hyper Threading is used, each core can act as two. In this case you will see four computational cores.
The EveryMac page for your MacBook Air explains why you are seeing four in some situations and two in the System Information application:
This model is powered by a 22 nm, 64-bit Intel Mobile Core i7 "Ivy Bridge" (I7-3667U) processor which includes two independent processor "cores" on a single silicon chip. Each core has a dedicated 256k level 2 cache, shares 4 MB of level 3 cache, and has an integrated memory controller (dual channel).
This system also supports "Turbo Boost 2.0" -- which "automatically increases the speed of the active cores" to improve performance when needed (up to 3.2 GHz for this model) -- and "Hyper Threading" -- which allows the system to recognize four total "cores" or "threads" (two real and two virtual).