Why could the mass of argon be greater than the mass of potassium even though it has a lower atomic number?

The mass given on the periodic table is the weighted average of the abundance of isotopes. Because heavier isotopes (more neutrons) of argon are more abundant than potassium's, by pure statistics, argon will appear with a greater mass on the periodic table.

in easy way

we can also say :- Argon has  greater proportions of heavier isotopes with more neutrons than potassium.