3.5 is what 4 Core max Turbo is (and that would be what you get in Prime 95). 3.8 is 1 Core max Turbo.
Interesting. I haven't noticed this because I have always run stress tests at an overclock with my 2500K. When you overclock, it forces all the cores to turbo up to the CPU ratio you set, right? But if you leave it at stock, it will do the 3.5 with all cores/3.8 with one core (with i7)?