40 hz vs 100hz, same speaker - which is better?

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I'm running the Onkyo Audessy set up. It's crazy to see the hz measure so radically different when the mic is moved an inch back.

Sometimes it lists the center speaker (which has has a passive radiator for bass), as 150 hz, sometimes 100 hz, and on rare occasions it lists it as 40 hz. I'm very confused as to which one better represents my speaker and what my aim should be.

Can any explain in layman terms what is a good vs bad reading when it comes to hz measurement?

Thanks in advance.