The extra threads on the Intel® Core™ i7 comes from hyper-threading and the best way to easly understand hyper-threading is to think of leftovers.
Rarely will an application use 100% of the resources of a processor. What hyper-threading does simply put is allow second pathway to come in and take advantage of the extra resources. This can give a small performance boost for multi-threading or heavy multi-tasking. Since hyper-threading doesnt run on 100% of the resources of the processor it will never have the full performance of a core but it can help on applications that can take advantage of it.