Two different displays for the same model

I've two Ipod Touch 4g, 8 giga and 16 giga.

Just recently I noticed that they have completely two different displays.

I bought the 16giga because of a fracture of the glass on the 8giga model about two years ago but only today I've noticed how they're different.
The panel on the 16 is a lot less defined and the colors are absolutely less beautiful.
I was just wondering how this could be possibile. To be honest I'm quite disappointed
(As you can see in the picture I'm not talking about details, the difference is pretty marked. Displays are both set as maximum luminosity)