Ugh! That happened to me only I found out at the doctor's my scale was about 15lbs-ish lower than I actually was! Imagine my surprise when I thought I was 239 or so and I was actually in the mid 250's! Horrooooorr!
I'd like to say three things though.
One, scales do tend to get off their rockers after some time... something about the weights inside getting worn out or something?... so an older scale will probably have more chance of being wrong than a newer one.
Second, not all digital scales are created alike. A bad digital scale is worse than an old analog one sometimes, I swear! I got a cheap digital scale and the thing was a piece of $#*&! I returned it and bought the Eatsmart scale from Amazon (about $30) that a bunch of 3FCers recommended to me and I haven't ever been so happy with a scale... it's never been wrong (compared it to two doctor's offices) and you can get on it five times in a row with the same weight! So basically.. it's all about the digital scale you choose.
Thirdly, and finally, just remember that you HAVE lost that weight, even if you didn't start at the weight that you thought you did. Changing your starting and current weights doesn't nix the fact that you've lost that massive and incredible amount of weight that you did!
So keep chuggin at it, invest in a GOOD scale, one you know you can depend on, and don't give up! You'll be where you wanna be in no time.