The graph shows the increase and decrease in stress when starting the 3Dmark test. You can also see an increase without lowering at the start of the game.
You've flashed the 64 bios, it has higher stock frequency, higher power limit, higher memory voltage and higher memory contoller voltage (the one that is shown as "memory voltage" in wattman).
The latter one is also being used as the minimum Core voltage limit for higher pstates of vega gpu. The default value is 1100mv.
In order to undervolt core at pstates 5-7 lower than that value you have to decrease "memory voltage" (memory controller voltage in reality) as well as core voltage.
Similar memory overclock stability is obviously not guaranteed with a lower memory controller voltage but usually at ~1000-1030mv it is the same.