Yesterday's number changed. Why?

Usually because a refund was issued today against an older order, which lands on today rather than on the original day, or because a cost rule was backdated deliberately, which re-prices the days from that date forward.

What should never move it is the sync itself. Closed days are recomputed only when something you changed asks for it, and the reason is always visible: either a refund with today's date, or a cost rule whose effective-from you set in the past.