Wood Flooring has its own natural color. When a coat of varnish or oil is applied, the color will change. The way the floor is sanded influences the final shade. A coarse abrasive makes the surface rougher. The varnish or oil will tend to penetrate deeper into the wood flooring than when using a fine abrasive and the color becomes darker. That is why it is important to use the same sand papers grit size for the main areas of the floor as for the edges and corners.
Practical tip: whatever grit you use for the main part of the floor, use the same grit for the edge of the room and corners