Because there is a general rule, in the vitamin "world" to take things with food to simulate "eating"

However, B12 is an exception. Its presence in the mixture if you take with food lessens its value since it will compete with the food for absorption. People with intact intrinsic factor will complex 2-4 micrograms the same as if it were derived from animal protein digestion. But the vast remaining amount of 1000mcg will not be absorbed most likely and will pass into the feces.

This rule I use is derived from research on thyroid hormone and digoxin which are given in microgram amounts orally as well. Studies show they are not absorbed with food, but are soaked up by the fibers and bulk of food going thru the intestine, and lost. Since B12 is the same dose, the rule applies to it. B12 is only absorbed in a small area of the small intestine. It is this fact...empty stomach or not, that has perpetuated the myth that B12 cannot be taken orally efficiently. It has only be recently that oral forms have been available in fact.

Many labels from supplement manufacturers contain erroneous information. One cannot trust them all therefore.

So if you choose to take the supplement mixture you have with food, the B12 may be wasted. The other things are less critical with this fact.
So if your goal is to increase B12 levels, you should take a separate stand alone B12 in addition --and do so correctly for best results.