My FCOM for the -800 says:
– – – – when N1 SET outer knob is in AUTO position and FMC source
invalid.
If the aircraft is shut down all the inputs in the FMC are reset for the next flight and if the outside air temp(OAT) is not entered, the FMC can not calculate the N1 and so you would see the dashes. This is my best expiation for it but others might have a different idea. We as pilots don't really notice things like this because once we set everything up properly and do the checklist all these little things should have sorted themselves out and we never see them.