2016 Frontier Fuel Gauge/ Fuel Warning Light

I recently purchased a 2016 Nissan Frontier. It runs great. All the literature that I have says that it has a 21 gallon fuel tank. The fuel warning light recently came on. The problem is that when I filled up the tank it only took 15 gallons. That makes sense because I had only gone around 273 miles. There is no way the fuel light should come on when the tank has 6 gallons left in it. Has anyone else experienced this problem?