3.93 (multiplier x21) can only be reached in a single thread with turbo boost. Your processor's real speed is 3.74.
Mine shows x21 most of the time under some load even with all cores used, but not when all cores are at 100%. (It shows X12 under no load.) When at 100% on all cores, it drops back to x20. When on one core only, it jumps up to x22.
full load on all cores: base frequency
partial load on more than one core or full load on 2-3 cores: 1 step up
only one core being used: 2 steps up
That's the way the turbo boost works.
It's kind of funny. With my current overclock, everything works great: 167 bclock, 3.33 base frequency, 3.5 under partial load, 3.67 single core use. However, if I boost my bclock up to 170, it will work fine until I have something using a single core. at that point, turbo boost pumps it up to x22 and the system crashes. I could raise some voltage somewhere, but I don't feel like it for now. base voltages are working fine for me, and I'm getting EE performance out of a $200 chip.