My i5 3570k is having one core much cooler than the other 3. I have a nzxt 120 cooler and I am pretty sure I installed in correctly and I used the thermal paste it came with. With prime95 at 100% the difference between the core temps is still about the same. ]http://[/urlExt]
I can't see your image but I know what you are talking about.
On my i5 3570k @ 4.2 Ghz OC, my 2nd core is ALWAYS 10 degrees Celsius cooler than my other cores. I have no idea why it does this as well, and I am pretty sure my paste and everything is fine. I don't think you should worry about it now unless something bad happens.
My 3570k at 4.3Ghz acts similarly, Core 2 is between 5-10°C cooler than the others. I'm using a custom water-loop if it matters.
Far as I'm aware this is fairly normal, that core might just be higher quality than the others. Wouldnt worry about it.