What you are describing is what engineers call "non-linearity". What that means is that the first quarter of the tank on the guage is equivalent to say, 4 gallons, but the next quarter of a tank is equivalent to 7 gallons.
This is not uncommon at all. Gas guages in cars are NOT laboratory instruments, but give merely approximate readings. Dont' be surprised if, when the guage reads "1/2" on a 20 gallon tank, that you it takes a bunch more (or less) than 10 gallons to fill it to the top.
Also, don't be surprised if two cars of the same brand, and same year, will show different results on their guages.
If you are really worried about how accurate your guage is, the best thing to do is to run the vehicle until the guage reaches a chosen point (say, 3/4 for example), and then fill the tank to the top. Record the number of gallons filled. Repeat this for other points on the guage (1/4, 1/2, etc.) This will give you an idea of how linear your guage really is.
I remember a Ford I owned in the 1970's had a guage where the movement from "F" to "3/4" was only THREE gallons. Of course, the movement from "1/2" to "1/4", on that same car, was about SIX gallons.