I&#x27;d wager that gravity isn&#x27;t 100% equal over the whole globe,  the pull of the moon&#x27;s gravity also isn&#x27;t equal over the globe, and that the circulation of water across the world&#x27;s oceans also isn&#x27;t 100% (so there&#x27;s different salinity, water pressure, various prevailing &#x27;currents&#x27;, temperature, etc...).  Of course I don&#x27;t study this, so I don&#x27;t know the exact reasons, but I&#x27;m sure there is a reason...