Economic, class, social mobility classically means ability for an individual to attain significantly more wealth than they were born with and ability to change their and their family's lifestyle. In which case it's not zero sum.

But yeah, some statistics indeed are just likelihood that the ranking order changes, or even self-reported...

It's like the definition of "middle class". Everyone thinks they're middle class. The OECD calls anyone with 75% to 200% of the average income "middle class". Classically the term means you are above the labour class but not noble.