as above, I do know similar effects, and sometimes ask me the same.
Are you carrying the same (other) FP (except beeps and SPL) with you all through this "experiment"?
Are these effects always the same, no matter on which FP you're working?
How do you find your day's best combination of SPL and SR for a swim of, let's say more than 400m? (...Stopwatch, calculation or feeling, or both a bit?)