I wouldn't worry overmuch about the difference between the Seefeld and Seefeldt spellings; it seems to be fairly common in German. (I have something similar in one of the lines of my family, where different generations of the same family are spelled Lischett and Lischedt in the records from the Heidelberg area.)
The coat of arms that you have, however, appear to have been borne by the Seefeldt family of Denmark. I have attached a .jpg of the three Seefeldt coats of arms from Rietstap's "Armorial Général" as drawn by Rolland and Rolland. The horizontal lines on the wide horizontal stripe mean that it should be colored blue, while everything else is white.
I suspect that some provider of "your family coat of arms" simply took this Danish Seefeldt coat and sold it to your grandparents. (This is not a new thing; I have examples of exactly this sort of "borrowing" one family's coat of arms to sell to another family with the same or similar surname back in pre-Revolutionary War Boston, Massachusetts.)