When we solve problems about capacity of such a can, we just use the inside measurements. which are the actual measurements of the contained liquid. Were you told that the radius was the outside radius, or are you just assuming that?
If an artificial classroom problem you are given doesn't state whether the measurements are inside or outside, just assume they are the measurements you need -- and state your assumption at the start of your work, to let the teacher know that you are aware of the real life issues but also know how to do the work. They are really just assuming negligible thickness for the sake of simplicity.
In a real-life problem, you would make sure you had the appropriate measurements -- either do the inside measurements yourself, or find out the thickness of the walls and subtract that from the radius, and so on.
It is a very old tradition that exercises for students are artificial and simplified, in order to teach the basic concepts without bringing in the complications of real life. It is a good thing that they do this, not a defect! At some point, it will be good to do a real-life problem or two, but it would be very discouraging for students if every problem were that complicated.
