There has been a theory for a while not that motherboard manufacturers tweak their products a bit to squeeze out the best performance of AMD processors. Basically, people have been seeing small performance differences and some have complained that the advertised boost frequency is not even reached.

The colleagues from hardware unboxed did some testing, and it indeed seems to be a case of motherboard manufacturers doing some tweaks with their products. They took the AMD Ryzen 7 3800X and tested it in a large comparison with 14 different motherboards. The same processor and cooling were used in each case and hopefully applied in a strict fashion as well. I've compiled a chart based on the data and this is the end result:

Cinebench R20 was run three times in a single-core test and that will read out the maximum boost clock monitored with HWiNFO64. The AMD Ryzen 7 3800X reached a boost clock of an advertised 4,500 MHz on a number of six motherboards. Then three mainboards hit 4,475 MHz and two 4,465 MHz. A total of three mainboards could not reach a higher boost clock than 4.375 MHz. It doesn't seem that the AGESA (firmware) impacts the boost clock much.

The worst performer is Biostar. It isn't uncommon to see performance tweaks, for example, all the big brands have optimizations for Intel processors as well, for example, the all-core turbo on ASUS motherboards with Intel processors. Likely they are doing this to get an advantage over the competition and to product position the motherboards a bit better based on performance. But this is an interesting finding none the less.

Ryzen 7 3800X Show Different Boost Clocks with different motherboards