It has no meaning whatsoever. Someone on the marketing side of the team decided that was a "competitive" number to present, outwards, and someone in engineering was tasked with, working backward from that number, coming up with some plausible calculation that resulted in it.
In the real world, they, like Azure and Amazon, will have single point in time outages that will wipe that out for a year or more.
Here is what an honest assessment looks like:[1]
"Historically (updated April, 2019) we have maintained 99.95% or better availability. It is typical for our storage arrays to have 100+ day uptimes and entire years have passed without interruption to particular offsite storage arrays."
...
"In the event of a conflict between data integrity and uptime, rsync.net will ALWAYS choose data integrity."
An outage affects availability, but as long as it's not permanent it doesn't affect durability. For example, if I add a new backup provider that stores data on-premise I've added a (nearly) independent data store. This substantially decreases my risk of losing my data unrecoverably (increases durability) but if I don't set up any sort of automatic failover I'm still at risk for substantial outages (no practical increase in availability).
For example, I don't believe Amazon has ever lost any S3 data (https://www.quora.com/Has-Amazon-S3-ever-lost-data-permanent...), and if they did it would be a big deal. Same with the other major cloud storage providers.
> Someone on the marketing side of the team decided that was a "competitive" number to present, outwards, and someone in engineering was tasked with, working backward from that number, coming up with some plausible calculation that resulted in it.
I would be incredibly surprised if that happened. That's not the way I've seen anyone work here.
(Disclosure: I work at Google, though not in Cloud)
Cloud Storage is designed for 99.999999999% (11 9's) annual durability, which is appropriate for even primary storage and business-critical applications. This high durability level is achieved through erasure coding that stores data pieces redundantly across multiple devices located in multiple availability zones.
Disclaimer: I work at GCP, although not in GCS specifically.