[gamegoblin]

You are confusing availability ("uptime") with durability ("losing data").

I could make a system that was only available for 1 minute every 10 minutes (10% availability) but never lost a single file (100% durability)

I could also make a system that was never down (100% availability) but would randomly lose 1 in 10 files (90% durability).

Common causes for availability issues are power outages, network outages (fiber cuts, etc), and DNS issues

Common causes for durability issues are software bugs, entire datacenter losses (e.g. earthquake destroys everything), or perhaps coordinated power outages if the system buffers things in volatile memory.