Missions & Complexes

Howdy. I've been going on a bit of a standings grind this last week to broaden jump clone options and lower market fees in certain popular stations. So far all the, not sure what to call them, but "6 hour bonus" level 4 missions (Dread Pirate Scarlet, Blockade, Worlds Collide, X Extravaganza etc) have been giving the same standings increase (7.1719% with social 5) across multiple different corps and running the missions in different security status systems. That is, until I started grinding Brutor Tribe. They only give me 4.7812% for the same missions and I cannot find any explanation for it. Not in a fleet or anything. All my google-fu turns up is the same old generic stuff. Is it something to do with the years of endless carebearing for this corp by missioners? The only thing I can think of is that the other corps I was grinding for were not so mainstream, perhaps not being worked by such a large population, or that the sveipar constellation is extremely high sec.