The difference can be in the amount of dipping that the piece suffers through.
In other words, these days, the dipper can choose to leave the part bathing for a short period of time and remove only a small amount of material hence maybe if any, a couple of pounds.
The guy that I use takes the time and patience to bathe the part according to what I dictate to him.
Example, my bare dash frame weighed 17 pounds stock. I said that I only wanted to remove a portion equal to just before half its weight and not at all in certain critical areas of course for strength. The dipper removed just enough based on timing to remove 7 Lbs total and the critical mounting areas were left intact. The Dash weighs 10 Lbs now.
The part has to be super clean and rust free or else, the solution will continue to enhance the rusted area and possibly destroy the part like you see in an episode of The Munsters. LOL.

Some fenders are kind of light right from the get-go, so dipping may not be so necessary other than just removing the undercoating and years of various paint jobs. One paint job alone under another along with lazy body work can weigh as much as 3 Lbs or more. Ask me how I know.