Positions at the front of the fleet are a bit weird. I'm now 208th but I notice they go strange when the fleet starts to round the mark. Not that it is easy to round Tangiers.
They may measure the distance on the great circle distance to mark? Somethings working though as I've climbed from 322 this morning to 111 now, but I did ask for a fast boat before the start.
I think they wait until you have gone a certain distance past the mark towards the next one before they recalculate. Therefore, in the early stages you are sailing away from the previous mark and back down the rankings.