Dxdiag result about CPU

The processor is hyper-threaded. Hyper-threads are not separate cores but instead allow a single core to run two threads simultaneously. It does not give the full performance of a dedicated core but instead uses the resources of a single core more efficiently. Hyper-threads are counted as a "core" by Windows though - that is why you're seeing "20."

See the Intel data sheet on that processor: Here. Notice the # of threads.