To me it seems this:
equipment was made in USSR, no buyers-sellers, producer didn't have to advertise it, could allow himself to write honestly what it is. And wrote quite pessimistically error by height 100 or 300 metres.
In reality trained controllers found their way with it, won't tell the plane its height, but will have a good idea where happens what in air and can see when a plane deviates from glideslope (with a mistake of 10 metres).
A military pilot there said he was in the habit in complex meteo-conditions to chirrup with the ground non-stop, initiating the talks himself, as every time he was telling the ground his height - the ground responded by distance and info type (you are) 20 metres to the left, and keeping this kind of short talk - number here - 2 numbers backwards helped him to get oriented where he is.