If the floor was the reference for both ends of the chalk line. If not it might be level.He said he chalk lines at the same height. If the floor's out-of-level then the chalk line's out-of-level.
I put mine at 20" for that reason. Only one person ever came in the house and said "why are the receptacles so high?".If I ever build a new house for myself, I will place the boxes 3' off the floor. Will save my knees and back