The dispersion rate is not ipso facto slow. At the point of origin, there are several factors to consider. The first is that the fuel wants to get thinner, and increase in area. After days, the film would be a good deal larger in area, and perhaps be mostly at its most stable depth: 2-3 mm. Immediately upon entering the sea, it is thick, never to be "deeper", However, it is also a stable mass, with integrity and a stasis point that will see it behave as if it were a floating ship. Only the top portions are affected by wind, the mass gives up its most shallow parts first. The body will have to deplete somewhat to reach its "largest/thinnest" state. Keep in mind that the part of the slick that is the oldest, is furthest from the origin, the mass left at the impact point would move with the current, slower rates than the prevailing wind. The smallest, sharpest point of the slick is the original impact point, that point at which the entire mass fell in to the sea, its weight overcoming its buoyancy almost completely. Consider the origin as a "smokestack", a source for the slick that immediately starts to widen as it disperses into a thinner and thinner layer.
Let me add at this point that I do not think the slimmer arm is fuel that surfaced after deep submergence. Instead, I believe it is simply a surface deposition that comes from a smaller (in volume) source. The most southerly points are the arrival at the surface of two separate volumes of fuel that originated with this aircraft, but from different areas of the airframe. The larger volume is perhaps most likely the area where both wings impacted, the smaller volume the area where the Horizontal Stabilizer hit. The HS contains roughly eleven thousand pounds of kero when full, the trim tank is assumed to have been loaded fully at this portion of the flight.
If the fuel simply sank a bit to ascend perhaps a thousand metres away, there would not be such a focal deposit of fuel. It most certainly could not have sunk far further, kero rises in sea water far slower than air bubbles, and air bubbles could take several hours to surface from 8- 10 thousand feet. I also believe that if sunk, the kero mass would not have remained small, it would branch into several rivulets, to appear at the surface in a far larger deposit, and much thinner in depth.
This of course is in conflict with BEA's initial conclusion of "En Ligne de Vol" and intact at impact. So be it.
If I was unclear, consider: the mass of fuel beneath the surface is drifting with the current, unaffected by winds, the increasing surface area is drifting and being blown by the winds. This is an additive that must be resolved to further pinpoint the impact point. In other words the distance from impact is not the length of the arms, it is the length of the arms Plus the 'current'.
bear