For clarification, the values 1,2,4,8,16,32,64 and 128 in the Flow Direction raster indicates that all cells flow into one and only one other cell. When one cell may flow into multiple other cells, the value stored in the flow direction cell is the sum of all potential directions. This usually means that you will end up with sinks - a sink is an area where the flow direction is not precisely determined as going into one and only one direction. When you fill the input dem, you are eliminating the sinks and forcing the water to find a way to flow out. This is the dendritic scenario. Sinks may be real and we have ways to handle them - this is the deranged scenario.
Christine Dartiguenave
Esri Water Resources Team