it is: Areas on the wood floor where softer parts of the wood
appear to have been sanded more than other areas. Occurs between areas
of annual rings or between mixed species of varying hardness together
on a floor, such as in feature strips, borders and medallions.
Using the wrong angles while sanding.
Resand the floor using a slight angle with the big machine. A hard plate
or multi-disc sander may be needed on softer woods.