Because platforms credit conversions to the CLICK date, not the action date: someone clicks Tuesday, converts Friday, and Tuesday's numbers change three days later. Add processing lag (ads platforms run hours behind) and modeled conversions filling privacy gaps, and 'yesterday' is always a draft.
The practical rules: never judge the last 3 days, compare completed weeks to completed weeks, and expect restated history. Last week's report pulled today beats the same report pulled Friday.
This is also why day-trading your campaigns fails: reacting to this morning's numbers means reacting to incomplete data. Weekly rhythm for reading, monthly for judging.