Fish scale grain is caused by the tree's medullary rays -- these are part of the tree's vascular system. Not all trees have them, they are far more common in hardwood species.
Medullary rays run from the cambium layer under the bark into the heart of the tree, much like the spokes on a bicycle wheel. They are part of a tree's vascular system and transfer both water and nutrient around the tree.
When wood is quarter sawn, you are cutting through the meduliiary rays at just the right angle to cause the pattern to emerge (i.e. partially along the ray, perpendicular to the growth rings). This is the same phenomenon that gives both european oak grain and australian sheoak grain (allocasurina spp) its distinctive fleck patterns -- the same is true with the various species of lacewood that exist around the world.
Mefullary rays are not harder than the surrounding fibers in the tree's trunk. If you have a fish scale pattern on your blades outer layers, and the blade feels harder than normal, then something else is going to be behind it... it's not the medullary rays.