At one of airfields our company flies to, published minimum MACG is 5%. No mountains there, I don't know why exactly is it so high.
Let's assume that approach climb gradient, calculated by performance app in the EFB, or extracted from AFM tables is exactly 5%.
ATC or airspace requirements? Some SID charts which have higher than standard climb requirements often have a statement to that effect e.g. Nice. I reckon the same is true for some approaches.