What am I doing wrong here

Some variation is fine, but a 10 degree difference is pushing it. It may be your heatsink, your cpu, or both are not very flat, causing uneven pressure. Easy way to tell is to place a straight edge (e.g., metal ruler) across it.

More likely though is that you have a stuck or faulty sensor. Download Realtemp - it has diagnostic that it does to test for this.
 
That's just a 6-7c variation, like Zero82z said perfectly normal.
 
I have a pretty steady 8-10C variation between cores 0 and 1 on my quad. Variations like that are perfectly commonplace and not a cause for concern.
 
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