Gas Tank Question, less thank 11.9 gallons?

I just filled up my tank for the first time since I bought it from the dealership almost 2.5 weeks ago. The trip advisor said I was at 384 miles, and that I had 22 miles left until the tank was completely empty. With that said, I was on my last gas bar and it started flashing letting me know I was low on gas. When I filled the tank this morning, my gas tank filled up at 8.77 gallons. Wondering where the three other gallons went? Any advice anyone? Thanks in advance