How does this happen?

My battery is knackered, it drops from 100% to 1% in minutes, so I went through the support function, it ran a diagnostic, it said i needed a new battery, I booked an appointment. Apple reminded me several times about the appointment, by text and email. I went to the appointment and the helpful guy ran a diagnostic - guess what it needs a new battery. "Oh" says the man at the Apple store "we haven't got one in stock". How can Apple book me an appointment for a battery replacement and at no point check if there was one in stock before i drove to the store?