This is something I can't seem to make sense of, even after reading the ingredient lists.
The non-reduced-fat Hellman's dressing is marketed as "real mayonnaise", but reduced-fat Hellman's is sold as "mayonnaise dressing". I've also observed similar differences in labeling with other brands of mayo, such as Kraft Mayo.
Why is the reduced-fat dressing not considered "real mayonnaise", and what is the difference between these two? What defines "real mayonnaise"?
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.
:P.