I had something similar with an i7 2600K, when I started benchmarking I was coming up short constantly.
I had set my MoBo (Asus P8Z68-V_Pro) into power saving mode, and forgot about it, so the CPU was only actually running at the stepped speed of 1600Mhz, took it off power saving mode, and CPU went up to 3800Mhz and the benchmarks ran as expected.
Entirely my fault of course, but something to look out for.