So my "DTE" indicator is flashing and I supposedly have about 25 miles to go before hitting E. I stop to get gas and my fill-up is only 17.4 gallons. I was expecting something closer to the 21.1 max capacity.

So "DTE" actually means "distance until the E light comes on" after which there is still about 3 or 4 gallons still in the tank?

not exactly...the fuel indicator light comes on before you hit zero gallons. not sure on the exact mileage but i think the light pops up around 12 miles left according to DTE.

owners manual
"the distance to empty mode provides you with an estimation of the distance that can be driven before refueling. the dte is constantly being calculated, based on tha amount of fuel in the fuel tank and the actual fuel consumption."