Imagine that your length of wood is made of a load of straws all stuck together running from left to right. you want to cut a " V Shape " out of the wood as the cutter cuts downwards the straw below is longer and supports the straw being cut and the cut is cleaner, as the cutter moves upwards to form the other side of the " V Shape " the straws being cut are longer and supported by a shorter straw, the lonest edges get pulled away and cause the fluffing on the grains edge.

You can reduce the fluffing by using spiral fluted cutters for strait cuts but yet to find a spiral round over for rounding. try doing the same cut on some softwood and the fluffing will be 10 times worse. Edging 90* end grain is cleaner but remember to use a scrap block and the end of the cut to stop breakout. Have fun working with wood as it can be very rewarding when it goes right and warming when it goes wrong. I burn my mistakes.
Mike :)