Do all pure samples of a given compound have the same percent composition?
Why yes, they do, subject to experimental error.
A pure substance has a definite chemical formula. Correct analysis should report that formula faithfully. Of course there will be experimental error, of course there will be good and bad analysts, but one of the hallmarks of modern science is reproducibility. The results obtained in a laboratory in Chicago should be the same as the results obtained in a lab in Berlin or Beijing.