Say you give four minimally competent cooks the same recipe from The Joy of Cooking and ask them to follow it precisely. If they do, you probably won't be able to tell the results apart. By contrast, medieval recipes were written for experts, to give an idea of a kind of dish. The concept of uniformity isn't there. If you give four good cooks the same recipe from Forme of Cury , you can expect four quite different dishes.
Many medieval dishes have recipes in several sources.
Sometimes the recipes will be much alike, but equally often, they will vary widely in ingredients and handling. Even within a single manuscript, you may find several quite different recipes with the same name. Variation was the norm. Medievals didn't expect blind obedience to recipes, and the recipes they wrote can't be followed blindly. Meat is forbidden, but cheese and eggs are allowed.