Low Fuel Indicator Warning Light??

I've taken my iQ down to "E" twice, now. The first time, the fuel gauge was on the last bar and was blinking and it took 6.5 gallons to fill it.

This past tank, I drove it 310 miles and was sweating it worrying about running out of gas. But it took 7.5 gallons to fill it so it really had a gallon left.

But WHY doesn't the low fuel indicator light come one? (I have two other Toyotas and the warning lights work just fine.)

I usually don't drive my cars down to below 1/4th of a tank but since this is my commute car I'd like to know my limits and WHY the fuel warning light isn't coming on. (Maybe time to have it checked at the dealer?)