Because the tide range at the eastern end is bigger than the western end, at about an hour before high tide the eastern end overtakes the western end. This causes the tide to run east to west with the tide still rising.
Camper and Nicholson on the "cnmarinas" web site used to have the round the island (IoW) tidal charts on an hourly base. Then they were taken over by Premier Marinas (I think) and they've been discontinued.
So that's progress for you /forums/images/graemlins/mad.gif