Your processor seems to be right on par with other Core i5-3570K processor owners with same or similar results here in my humble lab. The i7-3770K varies as well though though much higher temps: Core 1/2 at 38C, Core 3 at 35C, Core 4 at 32C.
The reason you're seeing those low varied temps is the power management and Intel Speed Step at work. Since you don't need Intel Turbo or all the cores for anything, it throttles the VCore way down resulting in lower temps. You're perfectly fine.
It's under full load you might want to check the temps to make sure they're staying around 60C or so. That's how you know you have a pretty decent chip. Hope this helps.