When creating a custom field, the "Instructions" are there to aid a user filling out a custom form. It's additional information for those who don't entirely understand the purpose of the field. When the field is a calculated one, the mouseover of "Instructions" on both the back- and frontends show the "Calculation" content. Why would it (need to) do that? It's not only confusing for the frontend user but it also weakens the sincerity of "Instructions" offered on other fields.