While there are certain areas designated as extended coverage in the coverage map (no Fido/Rogers cellular towers; ie parts of Saskatchewan), a device might connect to Fido-EXT simply by being outside of Fido/Rogers coverage, but within another provider's coverage -- even within areas designated as Fido/Rogers coverage on the map. These spots of Fido-EXT occur because the cellular towers of the different providers are not located together. Each provider have their cellular towers strategically placed. That is why I prefer to use cellular tower maps rather than the coverage maps.
~ taken from here.
There may be locations where a phone might not get reception from Fido/Rogers, but get better reception from the other networks. For example, the area around Petrolia and Dresden has more sparse cellular towers. It's possible a phone may occasionally connect to Fido-EXT between the towns.
**edit** Note to Fido: You might consider changing the colours on the coverage map. The pale blue and paler blue are rather difficult to discern from lakes and each other.**
Hope this helps