When I got the 11.16 XC service from Derby to Newcastle this morning, I sat in a seat which was displayed as reserved from Sheffield. I had my eye on the double seats opposite which were reserved from Reading to Sheffield. On arrival at SHF, the seats were vacated so I moved over and sat in one of them, needless to say, a minute later, both seats were claimed.
Presumably then, the display changes when the train comes to a stand in the platform? But you don't know what's likely to be displayed unless you stand there and watch it (which isn't always practical) Paper labels usually show the points of reservation for the whole route but electronic displays don't seem to have such a capability. So a reservation can pop up with no notice, not even ten minutes?