If HDR10+ GAMING display is certified, why does image quality vary between displays?

Display variation is a reality of the hardware, not a flaw in the standard. Different panels have different peak brightness levels, black levels, and color gamut. This means that a 3,000-nit OLED and a 600-nit IPS monitor will always look different, regardless of which HDR standard is used.

What HDR10+ GAMING certification guarantees is that every certified display correctly communicates its capabilities to the game engine and renders the resulting signal accurately. The game adapts to what your display can show, so you always get the best HDR your screen is physically capable of.