Daniel;
Flow accumulation grids are generated from a 8 point flow direction grid. I have downloaded one of your rasters and I see that the values range from 1 to 255. Starting from the East, the values for the flow direction grid should be 1, 2, 4, 8, 16, 32, 64 and 128. I suspect that the flow direction grid that you have uploaded were generated from an unfilled DEM, thus explaining the values that I am seeing.