How did you get per core info? Because I can only get motherboard CPU temperature and package temperature from CPU, not core temperature. And I would love to know.
As for temperatures, my is running even hotter, it goes all the way to 90C, spiking to 96C on AIDA64, but dunno about cores. VRMs are few degrees cooler. I have 2600X as well on Gigabyte Ultra Gaming motherboard with stock cooler. Also I tested with Ryzen Master, HWMonitor, Gigabyte motherboard utility and AIDA64, though latter was showing motherbaord readout, since it was about 2-3C lower.
Though I would say your difference is likely to do with different quality of cores. In Ryzen master, your best core will be marked with golden star and second best with silver circle. So those two will likely run cooler. I saw this on my old i5 6600 too, when core 1 would be the "hot boy" and would be thermal throttling and core 2 would be like 10-15C cooler and literally not care about anything. So it could be down to silicon quality. Still, if I can get per core temperature, I would love to test this further. But for now, I ordered few new fans for the case and hopefully that will improve temperatures, since XFR is pretty aggressive and loves to press my chip really hard and hot VRMs next to it doen't help with that, can't imagine putting graphic card into it. It would either kill something or I would have pretty strong room heater. :-D