It may be the labels are generic and incorrect. Not sure it applies to all missions but take charity missions vs normal source / deliver missions. Both say Med and ++ / -- as OP says.
But the one charity mission returns far, far more rep or influence with the local faction than any one equally labelled non-charity mission. And it seems the higher the credit value asked for, the higher the rep goes up with the local faction.
I haven't been focusing on charity missions like back in the days when I was empire Baron rank grinding, but whenever I see one I take it and it sure seems like a single charity mission with about 200-300k credits asked for moves the local faction rep bar up quite a lot compared to any single med ++ / -- equivalent labelled mission