Well, first of all, allow me to congratulate you on the 70 lb. loss.
One thing that might be different is how you carry yourself (body language, how you speak). I've only lost about half of what you've lost and friends have told me that I'm different, more confident. A change in demeanor can lead people to not recognize you right away; it doesn't always have to be based on appearance.
Also, you may be down to the same weight you were when these people last saw you, but your proportions might have changed (at least I know mine have) and that can make a difference too, even if the changes seem slight to you.
Whatever it is, please don't let it discourage you. Also, someone mentioned just asking them what they meant; that's a good idea too. Just a casual response of "Really? What's different?" is all you need to get your answers.