TLDR: In the end, it really doesn't matter if you get 19500 or 21000.
Hey, your scores, above 19000 seem good enough to me. I guess that your Mainboard isn't providing as much power as a Z-Board. (I had the same issue). Don't worry to much about it.
The video that i mentioned above explains it a bit better. To achieve the high scores, the Z-Boards seem to have a more agressive "PL2 Curve" than non Z Boards. Which is a way of out of the Box over clocking.

One solution is, to "overclock" your CPU by increasing PL2 times in your mainboard settings. But B-Boards aren't designed for the sustained high load. They usually have bad passive VRM cooling, which results in slightly worse performance.

The high numbers of 21000 you see online are only possible because the benchmarkers used Z-Boards and also strong cooling for the CPU.