gas tank only 13ish gallons???? missing 3-4 gallons

So having read that some people found their aftermarket fuel gauges to not be accurate I decided to drain the tank today gauge was reading 1/4.
I have a fuel filter fitted not far from the tank just before the line runs along the bottom of the chassis rail so figured if I siphoned from there when it stopped tank would be empty as far as engine being able to run.
Anyhow gauge works fine empty is empty so having got just over 4 gallons out of it went and filled tank at station only managed to get a further 9.5 gallons in total of 13.5ish I thought they where all 16-17????