Capacity vs velocity
Two numbers teams confuse: velocity is how much a team has historically completed per sprint, while capacity is how much it can take on next sprint given actual availability, PTO, holidays, on-call, meetings. Planning to last sprint's velocity when half the team is on leave guarantees spillover; capacity adjusts the historical average for the specific sprint at hand.
Capacity and velocity answer different questions and get conflated constantly. Capacity is how much time the team actually has next sprint, after PTO, meetings, and on-call; velocity is how much work it has historically completed. Good planning uses both, velocity to estimate what's likely, capacity to sanity-check it against reality. Using velocity alone ignores that next sprint has two people on holiday; using capacity alone ignores that raw hours never convert one-to-one into delivered work.