I always thought the reason Xbox games sell more is because Xbox focuses almost solely on the US Market and the US Market is easily the biggest market in the world, outshining Europe and Japan put together, where the PS3 rules.
My Console Library:
PS5, Switch, XSX
PS4, PS3, PS2, PS1, WiiU, Wii, GCN, N64 SNES, XBO, 360
3DS, DS, GBA, Vita, PSP, Android