IMO
its in the wort. theres a ton of minerals and nutrients in extract as opposed to AG
thats why water not nearly as important for kits than for AG .
from professor google:
In extract brewing, water is considered less important compared to all-grain brewing because the malt extract already contains the necessary minerals and salts from the water used during its production, meaning you can use relatively neutral water like distilled water without significantly impacting the final beer flavor, while in all-grain brewing, the water chemistry directly affects the mash process and therefore the overall taste of the beer, requiring careful adjustments to the water profile based on your desired style.
so possibly the extract has more nutrients in it that the yeast enjoy as opposed to whatever water salts and grains you are making into wort.