Hmm... seems like there's a lot of people that have this idea but no one knows why they have it at this point lol

I've thought about it personally and I can't think why anyone would logically think a pocket QB should receive 20% less RTP calls. If anything I almost want to say a QB running out of the pocket seems more likely to get hit below the knees, spun down, grabbed from the neck, hit in the face, etc... defenders are trying everything they can to grab the guy. Meanwhile, a pocket passer is usually just standing there and you can generally just tackle him.

Maybe a separate bias that would need to be explored are, do refs throw less RTP flags on running QBs because they're more athletic?

I just can't imagine the whole disparity explained away in terms of just running/pocket QBs but by your original comment being the most upvoted, many feel that is a possibility.