Weird Gas Tank Issue..

I'm a bit lost because I haven't heard of anyone having this issue before. From what I know, all 2013 Tundras have a 26 gallon tank. However, when I fill up (pictures linked is shortly after the fuel light came on), I can't even get close to 20 gallons in the tank. I know that it's not an odometer issue, as I've purposely ran the truck out of gas before, and even that fill up was 5 gallons away. I know in many vehicles there is always a gallon or so left in the tank that is "unusable" but is this just normal with these gen 2s? My previous '08 V8 didn't have this same issue, which is why I'm scratching my head. Should I just go with an aftermarket tank?

 

That is a little weird considering the 08 and 13 have the same size tank, 26.4. Tundra's have a rather large reserve to keep the pump submerged for cooling reasons, so I would think its completely normal to have a 5 or 6 gal. reserve till bone dry.

 

Can confirm the large reserve from E, I've only tested this once with a full 5 gal can when I first got mine. Tested it 3 more times while I figured out my fuel gauge was off or stopped working at a 1/4 tank.. Other explanations I've read is the reserve is useful for when you're towing, safe buffer zone to get gas.