It's not that Nvidia outsells AMD, it's the margin you see on steam that's odd. JPR's discrete GPU market share reports have had AMD pretty consistently around 20% from the launch of Ampere and RDNA3 up until the last couple quarters and the launch of Ada. There's only 3 RDNA3 cards on the Steam survey though, the 6700XT, 6600XT and 6600, for a total of 1% market. The list goes down to 0.15%, so even if all the others add up, total % must be under 1.5% or so. Compare that to the Ampere desktop GPU (ie exclude laptop one) and the total is 28.21%. So JPR shows Nvidia selling 4x more dGPUs during the most recent generation than AMD, but Steam shows 20x-30x more current gen Nvidia cards being used on Steam.
It's not impossible that both could be accurate and people buying AMD GPUs just tend not to game much, but it is strange and makes people question it.