I was testing out my i5-3570 (non-K) and noticed the exact same thing. My CPU has a 3.4 GHz base just like yours but can "boost" up to 3.8 GHz.
Yet, with all cores pegged in prime95 they only reached 3.6 GHz, just as yours did—and I saw this behavior not only in CPU-Z, but also in several other monitoring programs, including the official Intel "Turbo Boost Technology Monitor" (which sucks, btw).
Well, it turns out there's nothing wrong with our BIOS settings, or the reports we're seeing. It's really just confusion/obfuscation about what Turbo Boost actually does.
According to my understanding (as informed through several hours of Googling), Turbo Boost operates in "steps" depending on how many cores are active—meaning how many cores are being taxed upon.
For Sandy Bridge/Ivy Bridge architecture, each "step" boosts the frequency of an active core by 100 MHz (or 133MHz for earlier Nehalem architecture).
Our particular i5s have a stepping pattern of 2/3/4/4, each step signifying the number of 100 MHz "boosts" that are available to four, three, two, and one active cores, respectively.
Ergo, our max clock speeds, with Turbo Boost fully functioning, are as follows:
2 x 100 MHz for four active cores (3.4 GHz + 200 MHz = 3.6 GHz across ALL FOUR cores)
3 x 100 MHz for three active cores (3.4 GHz + 300 MHz = 3.7 GHz across THREE cores)
4 x 100 MHz for two active cores (3.4 GHz + 400 MHz = 3.8 GHz across TWO cores)
4 x 100 MHz for one active cores (3.4 GHz + 400 MHz = 3.8 GHz across ONE core)
So, there you go. The max Turbo Boost of 3.8 GHz only occurs when ONE or TWO cores are active/being taxed.
When we're benching or stress-testing with all four cores, like we do in prime95, for example, we'll only be able to push all four cores to 3.6 GHz. Which still isn't bad, I guess, but It'd be nice to get the 3.8 across all 4 cores for a limited boost, then maybe drop back down to 3.6 if the load persists and temps get too high.
I don't really blame "we the people" for misunderstanding this technology. If you take a look at Intel's official Turbo Boost page, it doesn't explain the specifics of the technology very well at all. It's very vague, though the picture of the rocket flying out of the CPU tachometer is helpful (ha!). The Wikipedia page for Intel Turbo Boost is much better, though outdated. Online stores usually just say "Turbo Boost 3.8 GHz" which is slightly misleading because you kind of assume by the way they phrase it that you're going to get 3.8 GHz across all cores.
2x100 MHz for four active cores (3.4 GHz + 200 MHz = 3.6 GHz across ALL FOUR cores)
That part is something that I didn't even know.
A 2500K is 3.7, 3.6, 3.5, and 3.4 (1, 2, 3, and 4 cores). The 3.8, 3.8 for 1 and 2 cores is a new thing to me, and actually explains why I've seen 3.6 is the max 4 core Turbo for a 3570/K, when I would've thought it would be 3.5, going by how the 2500K would be.
Yep, I already found out for myself few days after posting this but damn this really is in-depth details perfectly explained!
I think the best thing to do would be just to OC the proccy manually via Bios to 3.9 or 4.0GHz like I have done at the moment & keep the Enhanced Speed Step enabled & keep all other features just like they were.