Yeah I agree that's annoyed as hell. The only thing I can think of (besides whatever's hidden we don't know about) might be if one is queues for roulette and the other slaughter only. I've done roulette to try and grab the daily poetics bonus and had others bypass me while my queue time fluctuated because it kept swapping me between Slaughter and Secure which I could tell was the case based on the time estimate fluctuating. I'm pretty much convinced that In Progress joins are Last Come First Serve which I don't like.