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My WAG is that some of the people who are reporting and tracking cases fall behind, and get caught up towards the end of the week.
One would assume the tracking process to be fully automated, but it is not; which is why there are different numbers at different websites. Some of the data appears to be derived from local news reports gathered up manually and entered into various systems. Then there is some attempt by some of the systems to check out sources they may have missed on other systems and bring them all into approximate reconciliation.
It's "Science" don't ya know?
State of Michigan admits that Thursday night is “roll-up” to enter the hand-written reports (lagging) from around the State.