Why did a past day recompute?

Because something asked it to. Either a cost rule was saved with a past effective date, which re-prices from that date forward, or a refund was issued today against an older order, which lands on today rather than moving the old day.

A sync on its own never rewrites a closed day. If a figure moved and neither of those two things happened, that is worth telling us about rather than living with, because a statement that changes without a reason is not a statement.