Actually what you are saying points towards it being a server wide percentage over a specific period of time.

Because on a newer server you would have less people doing the upgrade then and older server with more people actively doing upgrades. SO you could be seeing 5 out of 50 fails which is 90% on a newer server while on an older server we have had 5000 fails out of 50k attempts. SO those 45k successes make people complain alot less