A few (mainly OLED ones) are using 1.5 pixels when you would normally have 2. So they display 2880 real pixels and extrapolate the 4k down to that. I think they get away with it because they do have a "half" pixel. They alternate the colours green and blue every 2 pixels, but each pixel does not do the full colour range. Similar to the iPhone retina display.
A bit like how digital video has a lower colour resolution to save on bandwidth, these tvs may have a lower sub pixel resolution to save on costs.
It basically tries to use sub pixel rendering to make it look like it is a higher res panel. :/
I know it's pedantic, but I prefer them to advertise the true details of the panel. Far too many just go "it's a HD TV" and later you find out they redefined "HD" to mean "Heavily Dented". ;)