digital vs. mechanical scales

I've been playing my Wii Fit almost everyday for the past two weeks. I just played it and it said I've lost 20 pounds.I didn't believe so I stepped on the mechanical one in my moms room and I didn't lose any weight. So I restarted my Wii, thinking there was an error. And stepped on it again and it said that I lost 20. I certainly don't look like I've lost 20 pounds. So which one should I believe? The digital or the mechanical?