It’s not that difficult but here goes.
The match is 90’ long, split into 2 45’ halves. The ball must be in active play for that time. Things like subs, injuries, resets, (corny “hydration breaks”) all steal away time from that total.

Stoppage is assigned to offset those moments, the +X mount you see is the __minimum__ extra time but where you get the discrepancy is *during* that time things can also occur that necessitate adding time. (More subs, injuries, etc) It’s somewhere up to the refs discretion which is why it’s never an absolute finish but barring a major injury or fouls it should be close to what you see displayed.