Core i5 Processor running too fast?

It looks like a combination of Asus Autotuning plus Intel Turbo Boost.

Turbo boost will cause three of your cores to sleep, but boost the clock speed of the remaining core. In some cases, this is more power efficient than having multiple cores running.

The i5-4670K gets 4 boost "steps" in 1 core mode. With a 3.4MHz at normal:

3.4 + (100MHz * 4) = 3.8GHz