HTC One X - 720 x 1280 pixels, 4.7 inches (~312 ppi pixel density)
HTC One S - 540 x 960 pixels, 4.3 inches (~256 ppi pixel density)
The One X is using a crapload more pixels to display the same icon, of course it look better when compared 1:1 with the One S. I also wonder about a crapload of other settings they could have manipulated.
Not arguing that full RGB is not better than pentile, but it is not that better at high DPI.
If the One X picture would be a bit more high resolution, which it should
be for a fair comparison, you would see the subpixels. The mere fact that you can't see them should set of flags that they ****ed something up.