It's as Spitoon says, and can be very common depending on the radar equipment and its useage. In particular, area controllers operating primarily above the Transition Altitude and well clear of TMA operations have no real need for such a conversion from the reported Flight Level transmitted by the transponder to an altitude so it might not be displayed as anything except the former.
Imagine you are flying along at 900'AMSL quite comfortably on a QNH setting of 1043HPa (not unknown in the UK). Any radar without altitude conversion will be showing your Mode C as 000 ..... because with reference to the standard datum of 1013HPa, that's where you are. Now you drop down 100', and the radar will show you at -001, and so on. But amazingly you are still above terra firma. Similarly, if the pressure becomes even higher, then so will your indicated Flight Level on an unconverted radar screen.