According to the graph above it seems there are plenty of them closing down in the real world and hence the value on the spreadsheet will most probably be floating above and beyond 100%
It will never be zero off.
In Portugal I have the official list of stations and I've been following it and know the amount of specific brands that are still missing on waze (also missing: 48h per day), but from time to time I see double landmarks created by newbie users albeit I don't understand how it happens besides stubbornness - my landmarks are snapped to the roads around so they are as big as possible and cover the whole station land, and some newbie editors think that the landmark should cover only the strict rain ceiling above the gas pumps, so they see my landmark locked and create a new smaller one above it. I've seen this only happening less than half a dozen times and the editors understood the mistake, but I've said this just to explain that it's indeed possible to get more than 100% due to these duplicate stations.
There is also the possibility of ghost stations or DB corruption
see my direct champs thread where I have two stations on the client but only one on the map, for many months now.