Remember - at the end of the day - Wii Fit+ the balance board are nothing really more then a game/game accessory. The balance board is shockingly inaccurate when it comes to weight detection to be honest about it.
Two occassions spring to mind...
shortly after I brought it - I got one of the silicon covers for the balance board - I had used it the night before to play Wii Fit, and after putting the cover on it and standing in the spots suggested by the cover - I had apparently lost 7lb in about 24hrs. (I wish)
Another occassion - I used it to judge how bad Christmas just before the weigh-in following Christmas Day - went on it 4 days later (after having a BAD few days) - I had apparently put on (from wii fit session before weigh-in, not my official weigh-in weight) - 13lb. (at next weigh-in - the gain was 4lb).
Guess what - I don't use it for weighing myself anymore.