I think it'd be worth calling out the differences.
Either both should have the magnifying glass or neither. This just makes it hard to see the difference.
Maybe it's the inconsistent lights/shadows?
Maybe a pixel artist has the proper words to explain the issues
1 - AI just try to compress too many details into so few pixels.
When artists create pixel art they usually add details along the way and only important ones because otherwise it will look like rubbish on some screens.
Also it's easier to e.g add different hats or heads or weapons on the same body. AI generated ones is always too unique.
2 - AI try to mimic realistic poses that look like art supposed to be animated in 3D.
For a real game if you make lets say isometric tactical game you'll never make tiles larger than 64x64 because of how much labour they will take to animate. Each animation at 8fps take hours of work.
So pixel art is usually either high-fidelity and static or low-fi and animated in very basic ways.
Generated pixel art for now is 80-90% done state. To use them in prod, issues should be fixed which seems to be the palette and some semantic issues. If you only generate small parts of the big picture with AI, it will be perfectly usable.
Are you talking about the LoRA by LuisaP?
Somewhat ironically, that LoRA's showcase images themselves exhibit the exact issues (non-square pixels, much higher color depth than pixel art, etc) that stuff like this project / unfake.js / etc. are designed to fix.
On the OpenAI side, the gpt-image-1 model has actually had the ability to produce true alpha transparent images for a while now. Too bad quality-wise they're lagging pretty badly behind other models.
Or is it purely because the models just don't understand pixel art?