Availability is often confused with reliability, but they measure different things. Reliability (MTBF) is about how often something fails; availability is about what fraction of scheduled time it's actually ready to run — which also depends on how fast it gets fixed (MTTR) once it does fail.
That's why two assets with identical MTBF can have very different availability: the one with a well-stocked parts store and a technician nearby recovers in minutes, the other in hours, even though both fail equally often.