The atomic masses in the Periodic Table are not integral numbers. For example, phosphorus is 30.97 instead of 31. Current technology does not allow for exact measurement of such a small quantity.
Atoms gain and lose electrons easily and that changes their masses significantly.
Atomic masses listed in the Periodic Table are weighted averages of isotopic masses. Atomic masses are measured in real samples that are always contaminated with other elements. There is a theoretical uncertainty in the masses of atoms. Each element is composed of tiny, indivisible particles called atoms. All atoms of a given element are identical to each other, but different from those of other elements.