I'm 100% certain that it's due to added salt during cooking.
In fact, look at this entry for cooked carrots
. If you scroll down to the bottom of the page, you'll see this:
Contains these Ingredients:
146 g Carrots (without salt, drained, cooked, boiled)
4 g Margarine (regular, 80% fat with salt, stick)
0.9 g Salt
(Margarine? Really?! YEEEECH!)
My advice is to always just use the raw measurements.