If argon potassium and calcium have naturally occurring isotopes that have the same mass number 40. Explain how this is possible?

Isotopes of an element have the same mass number but differ in the number of neutrons. In the case of argon, potassium, and calcium, each has isotopes with a mass number of 40, but they are different elements with distinct atomic structures. For example, argon-40 has 18 protons and 22 neutrons, potassium-40 has 19 protons and 21 neutrons, and calcium-40 has 20 protons and 20 neutrons. This phenomenon occurs because the mass number is the sum of protons and neutrons, allowing different combinations of these particles across different elements.