It might depend on the "serving size". The Wendy's salad dressing would be the carbs for the whole container. But the bottled dressing might be zero carbs per tbsp. HOWEVER, by law,that label can say ZERO if there it has "less than one carb per serving." Even 0.9 gm can be listed as zero.

Or perhaps one dressing does just have more carbs. Read the label. The first ingredients ought to be low carb things such as oil, egg yolk, vinegar, spices. Sugar and starch ought to be 5th or more on the list, or else go looing for another brand. Creamy dressings TEND to be lower carb than other types. So I make my own.

I don't know where you live, but in Canada there is a brand called Renee's Gourmet Dressings which are very low carb and delicious.