My recollection is that Ilford changed the formulation of Delta 400/400 Delta at one time during its marketing life, if only a little. Given that this emulsion has not been available in sheet film for some time, it may be that your 5x7 sheet film is the earlier formulation. The recommended dev times are not quite the same. You may be best contacting Simon Galley at Ilford for clarification as to which year/batch matches your film.