Could be dirty contacts on the rotary switch or it could be a faulty LED decoder/driver chip or it could be faulty voltage dropping resistors feeding voltage to each segment or it could be a faulty LED display or it could.........I think you get the picture. I have never noticed a "usual" cause but all of the above are quite plausible as is faulty solder connections on the switch or LED itself or the connecting wiring harness that plugs into the display board if the unit has one.
