One, Mass and weight aren’t the same thing.
Two Relative atomic mass is the mass of the ratio of CL 35- CL 37. It has no meaning outside of the expected ratio which can very significantly.
Take it up with the IUPAC. I am done with you and your refusal to go google Chlorine.
The IUPAC definition of atomic weight is:
An atomic weight (relative atomic mass) of an element from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of an atom of 12C.