...And...
They might be testing trends on pay expectations.
Bingo! By Jove! I think you've got it with those two explanations.
My estimate is that less that 50% of advertised jobs are ever filled. There is no intention of filling them. They are just wage and salary surveys.
I spent way too many years working at "Bigtime Mega Corporation." Saw this kind of stuff at first hand. It has been going on for quite a while.
I think you are correct.