I can understand that.....and completely relate. It's either one of two things. Either the factory "rounded up" (quite inaccurately) the original tank's capacity trying to compete with the competition, OR the new tank had corners cut when it was designed and made. What brand tank is it?
I don't recall the brand, other than to say that the replacement tank looked identical to the factory original tank when I had them side by side on my driveway.