Heather:
I think a lot of emphasis when grading a resort is put on the room and what facilities are contained therein. Coffee maker, cd player, iron and ironing board, mini fridge, in room safe, bidet, hair dryer all have a large effect on determining the rating of the resort.
Playa Pesquero has 7 a la carte options and 2 buffets. Melia Cayo Santa Maria only had 1 buffet and 2 a la cartes. Playa Coco has very little to offer at all apart from 1 buffet and 3 a la cartes. All three are rated 4 star plus resorts, but the PP is head and shoulders above Playa Coco, I don't think anyone would argue that. The rooms however have similar amenities and that is why, IMHO, they are rated the same.