I saw this very thing with a Zoop this past Wednesday. My partner and I were diving for bugs at Pompano Beach and had gotten some 31% nitrox to lengthen our NDL. So I set my Petrel to a blend of 31 and went ahead setting up the rest of my gear. My dive buddy did NOT set the blend on the Zoop to 31. After a good 45min dive we start the ascent and the Zoop starts going crazy and beeping and so forth. Nice big Er on the screen and my buddies eyes are about as big as saucers. On the surface, I asked him if he set his computer to 31 or had he left it on 21. One guess what the answer was.. He relearned the valueable lesson that was taught in his Nitrox course.