Why does Network Restore on my MacBook Air 2012 give me Mountain Lion, not Mavericks
Internet Recovery will only download the original version of OS X that came with the computer. This is not configurable.

Back when Internet Recovery was released, OS X updates weren't free, so Internet Recovery didn't know if you paid for the upgrade from Lion to Mountain Lion. Therefore it was easiest to just install the original OS X, and then make the user update afterwards.