Using auto-exposure? The white wall behind the dog probably fooled it. You really need to meter on the dark dog to get it right.

I love Delta 3200 and it is great at 6x7. I too often prefer it at 1600 but hand held indoors without flash it can be used at 3200 or 6400. DD-X for 7:00 sounds short to me though; I usually develop it for the next time up (e.g. shoot at 1600 and develop for 3200) but I like denser negs than some.