If the output is an expression, then the expression results do not need saving - they are computed from other saved signals. So the only choice is whether something is plotted or not.
If the output is a signal, then it can be saved (by the simulator) but not plotted - for example, you might want to plot it later, or use the value in an expression - which you then want to plot.
Put another way, the save checkbox controls whether the simulator saves that thing or not, and the simulator is not saving expressions, only signals.
Regards,
Andrew