Just because a piece of material is round does not mean it's straight (cylindrical). These are two different measurements and have absolutely no relationship to each other.
Put the material on a granite or other flat surface and check to see if you can get your thinnest feeler gage under either the ends or the middle of the stock. You could also set up a surface gage with a test indicator to check for cylindricity. https://www.gdandtbasics.com/cylindricity
One can also simply roll the material on the surface plate to see if you can feel it "flopping".
Put the material on a granite or other flat surface and check to see if you can get your thinnest feeler gage under either the ends or the middle of the stock. You could also set up a surface gage with a test indicator to check for cylindricity. https://www.gdandtbasics.com/cylindricity
One can also simply roll the material on the surface plate to see if you can feel it "flopping".