At first glance the 2 things are the same. Take a bike, load it up with what you need and ride it from place to place, spending each night somewhere different from that mornings starting point ... I think the differences are a touch more subtle than that though. To me, off road touring (ORT from now on) places a real emphasis on the final destination and the actual bike riding, in much the same way road touring does. Covering a certain mileage each day, staying in a predetermined location each night with a schedule to meet. B&B, hotel, hostel or campsite pre-booked and waiting at the end of a long day ... always close enough to the real world that you can jump back in for a hot shower or pub lunch.
I think that bikepacking shifts this emphasis and whilst the destination and riding is still important, it's the nights away that define a bikepacking (BP) trip. I believe that BP has much more in common with backpacking (that's proper backpacking, not lazy students having a year off) than anything else. Perhaps that's why there's such an interest in kit and and an almost overwhelming desire to minimise the weight of kit with BP. In my mind, where you choose to spend the night also plays a role in deciding whether you're BP or ORT. Self sufficiency is a big part of BP, so once you leave the 'trail' and head off to the B&B or pub you cease to be BP and have become an OR Tourer. If you roll out a bivvy bag, climb under a tarp or erect your tent right there at the side of the track then it's BP.
Trip duration or at least its importance could also be a factor. The word touring seems to imply at least a few days away. On the other hand, a valid BP trip might only involve a 10 mile, 2 hour ride out and the same back the following day ... remember the emphasis is on the 'night out' with less importance placed on the ride itself. It would also appear that how you choose to carry your kit could have a bearing ... BP does seem to have developed a certain style and has come up with solutions to problems that ORT doesn't seem to have considered.
Like I said at the start, these are just my thoughts and I'm pretty sure not everyone will agree ;o)