According to this wiki site, the iPhone 4 has 326ppi, the Galaxy Nexus 316ppi and the Rezound 342ppi. But I don't think that's the problem.

Also there is something about PenTileTechnology standing there, which the Nexus uses.

So in the end, it seems this is the reason the iPhone 4 produces a better picture than the Nexus.

But I don't know what the HTC problem is. Maybe the display technology? But a