Most scales are a bit different from each other. Ten lbs is a bit extreme, but my guess would be that it's probably the analog scale that's off.
My guess:
I'm not sure if this makes sense, but if it's anything like a car's speedometer... if you ever take a GPS into a car with you that accurately tracks how fast you're going, you might notice that your car's speedometer is in total agreement with the GPS until it hits a higher speed. Suddenly, the car will say you're going 80 and the GPS says you're only going 73... the GPS is always right. It's like.. the more speed, the less accurate and sensitive your speedometer is to the actual speed you're going.
I would presume that the scales would be the same when you did the little test, putting a few lbs on each... but when you put MORE weight on, one of the scales becomes less sensitive to the actual weight on it, and thus is inaccurate. Inaccurate by ten lbs!
I think your best bet is to choose one scale, forget about the other one... and stick to the number on it. Even if it's not totally perfect, you're still seeing losses based on it.
