Why does the SLA percentage in Statistics not match the number of breached tickets?

Because they measure two different things. The "SLA right now" numbers are a live snapshot of open deadlines at this instant, how many tickets are currently breached, due within 1 hour, due within 24 hours, or paused, regardless of the selected date range. The attainment percentages ("met on time") are verdicts on finished clocks, measured by ticket creation date over the selected period. An open, breached ticket has no verdict yet, so it appears in the live count but not in the percentages.