LME has a set gravity, because it is already mashed to a set OG. Fermentability indicates how far down from the OG the fermentation will go, i.e., the expected FG. It really has nothing to do with where you start, gravity wise, it just shows where you expect to finish.
All extracts are made from different grains. That wheat LME is made of wheat and pilsner, while the dark malt extract is made of some combination of pale malt/two row and something like black patent (which adds color, but does not add gravity). Each of these grains that make the extract have a different gravity contribution, so their extracted sugars are expected to be different in the extract.
Then, the fermentability numbers aren't linear/dependent on OG...the mash process creates both short-chain sugars that are highly fermentable and long-chain sugars that are not fermentable at all because the yeast can't metabolize them. You can have a higher OG due to a higher % of long-chain sugars and be less fermentable than a lower OG extract containing more short-chain sugars.
Lactose is a long-chain sugar that adds to the density of the wort (gravity), but yeast can not metabolize, so it dillutes/drives down that fermentability variable, but drives up OG and FG.
Anyway, that's the basics. None of those variables are directly dependent on the other, like you are assuming.
The good news is if you move to all grain, you can control all of those variables based on mash temps, whereas you have no control in the extract process beacause you are not mashing, it's already pre-mashed and reduced to syrup.
Let me know if you have any questions!
