I think I remember reading that it cost more to post to masterscould be part of it
also, seeing as its been a couple of years since the last assignment, there was probably a few more accounts that they cleaned out than in previous times
which could mean reduced hit totals if someone has to pay less for the task due to using masters
such as this post on reddit, that I just commented on...
https://hw.reddit.com/r/HITsWorthTu.../us_academic_research_study_15_minutes_derek/
a hit was $2.75 back in march, and non masters,
but is masters, and only $2 now