Most supermarket brands of olive oil are blended from oil from many different varieties, regions, and even countries. Because olive oil tastes differently year to year from the same grove due to weather, trained blenders must take oil from many sources and come up with a new mix every year to create a consistent taste. Many excellent olive oils are also blends of different varieties from the same farm.
Blending some oil high in polyphenols with one which does not will increase its shelf life.
Sometimes olive oil is blended with canola or other vegetable oils. This should be stated on the label. Illegal blending of cheaper hazelnut oil can be profitable for the unscrupulous and can be difficult to detect.