Bingo. She was probably running with a barred rock. Hens can carry viable sperm (is that the right word?) for up to a month after being with a rooster. I know this for a fact.
In reading back thru your posts, it seems these birds would be the result of your black orp hatch back in March that you posted about. Is it possible that you got these hens from someone else, and just started hatching eggs from them? You sounded like you were very new to chickens then.
If Im not mistaken, the barring gene has to express itself if its there, it doesnt "hide". Im sure the experts will correct me if Im wrong.
Edited to add: in looking at your roo again, perhaps he isnt double gened barred. He's pretty dark for that. Are you sure someone didnt play a joke on...