GSMArena decided to dig deeper to find out if it is true, and so they put the display of the Xperia Z5 Premium under a microscope. According to their findings, they conclude that it is possible that Sony might have opted to keep the phone at Full HD for certain times and 4K at others. This is actually not that bad of an idea especially when consider how much battery it could save.
It might also explain why the handset isn’t overheating with a 4K display that would require a lot of power. However a recent teardown revealed the use of dual heat pipes, although we suppose the swapping between resolutions could have played a part as well. However as GSMArena notes, what they are looking at is a pre-production unit.
This means that it is possible that when the phone is released to the masses, Sony will have “fixed” the display resolution at 4K. However at the end of the day, we suppose we really have to ask if one can really tell the difference anyway, or whether it is a big deal.