I think that the type of fuel sold in your area, along with the method of storage makes some difference.
Until recently our area of the country had "reformulated" fuel.....they did something with the fuel to make it burn cleaner - and I just don't think the shelf life is very good for this fuel. Here is a list of the areas that sell this fuel, and until last summer Campbell, Kenton and Boone County in KY were "Opt -In" states and volunteered to be included in an effort to get more Federal money for road projects
https://www.epa.gov/gasoline-standards/reformulated-gasolineI also believe that if your equipment is stored where the temperature and humidity swings are kept at a minimum. If you store your equipment in an insulated garage the fuel will keep longer than it will if you store it in a carport where temps and humidity have huge fluctuations.
It also can make a difference what kind of fuel you buy - economy convenient store fuels have far less additives than the top tier fuels that you buy at Shell/Sunoco/BP/etc.