Most likely the algorithms that chooses whose ad gets shown are optimized to not have to look up each persons spend for that day, as syncing that between all se

Most likely the algorithms that chooses whose ad gets shown are optimized to not have to look up each persons spend for that day, as syncing that between all servers on every page request would be more expensive than all other steps combined. I'm not sure how they could let you overrun by several times your budget for a day however, syncing these every hour shouldn't be a problem.