I agree. Its a bit confusing looking at this document link below. According to this document "Availability is measured at its steady state, accounting for potential downtime incidents that can (and will) render a service unavailable during its projected usage duration.: Which are these potential downtime incidents?
However when having a look at the SQL query that is used by TSSR.
WRT using PING to measure availability, will that not give you a false reading as well if the network is at fault vs the server being up.