Difference between "real mayonnaise" and "mayonnaise dressing"
According to Duke University:


  Believe it or not, there's a standard of identity for mayo.
  Mayonnaise, as defined by the U.S. Food and Drug Administration must
  contain vinegar, egg or egg yolks, and at least 65% oil by weight. It
  may contain spices and natural seasonings except turmeric and saffron,
  since yellow color might suggest added egg yolk.


You can also review the complete standard.

Therefore, anything that does not meet this requirement cannot be marketed in the US simply as mayonnaise.

Reduced fat dressings, do not have the same amount of oil (necesary to meet the standard), since they need to eliminate it to reduce the calories.   Instead they emulsify in additional water via stabilizers.