Flammability isn't a threshold you cross once. It's a window — and there are two ways to be outside it: too little fuel, and too much.
The Lower Explosive Limit (LEL), also called the lower flammability limit, is the lowest concentration of a fuel vapor or gas in air that can sustain a propagating flame — below it, the mixture is too lean, too much air and not enough fuel, to burn. The Upper Explosive Limit (UEL), the upper flammability limit, is the highest concentration that can still sustain a flame — above it, the mixture is too rich, not enough oxygen relative to fuel, and combustion can't propagate through it either. Between the two sits the flammable range, where a spark, hot surface, or flame will ignite the mixture and the flame will propagate. Methane, for example, has an LEL of about 5% and a UEL of about 15% by volume in air — everything between those two numbers is the danger zone; everything outside it, on either side, is not currently ignitable.