It seems that ARO is the holdup... I'm 12/1/2019 (day I submitted) and Biomathematics ARO.
This has been suggested before, but the dates make most sense to me as cutoffs: if you were updated in late Dec, jan, feb or early march, it probably means a cut. Reason for this, is at the next round, the reviewer only wants to pull up "active" applicants who are still in the running. But hey, it may just be an early Dec hopeful dream.
I'm thinking ARO for some reason just didn't put in updates yet, hence the high rate of early Dec (day of submission) updates.