As somebody in another forum mentioned - it really depends on the speed of reading of the different admissions reviewers, even if they go by date completed. Picture this:
If there are 1000 applications in stacks of 20 at a time given out to 10 different reviewers. One reviewer may have a planned weekend, so they rush and review their stack early, another one may be busy so they keep going later into the week. Once a reviewer is done with their 20 they grab another stack, so a small bunch of people who completed later are getting reviewed slightly earlier. Either way the process is relatively random but as we can see the date completed numbers are slightly moving up.
Another factor is - the apps per date completed are probably much more in later dates than earlier. The adcom mentioned that they get about 60% of their applications in the last 24 hours of the review period. As we can see, the dates completed are moving relatively fast now, but in the next few days we will likely see those slow down. If it took 5 days to see dates go from 13 to 20 it may take a lot more to see them move from 20 to 22.