why is argon heavier than potassium even though argon has lesser atomic value in rounded values than potassium - Chemistry - Pure substances and mixtures

Question

why is argon heavier than potassium even though argon has lesser
atomic value in rounded values than potassium?

na · Accepted Answer

This is simply because the atomic mass of K is 39 while that of
Ar is 40

this is due to the fact that the atomic number of Ar is higher than
that of K and hence there are more number of protons and electrons
in the nucleus of Ar than that of K

why is argon heavier than potassium even though argon has lesser atomic value in rounded values than potassium?