IN terms of your seemingly disproportionate leanness, you have to realize that there is a very wide discrepency in terms of how different people carry their bodyfat. Aside from where, some people also seem to conceal it much better (usually if they have larger bone structures, and more overal lean muscle mixed in).
Personally, I hate those little digital things. I've had them give me ridiculous readings a week out from contests when my entire body is a web of vasularity.
Most physique conscious folks rely on the mirror, because at the end of the day, no one really knows your bf%, or your weight, they just know what they can see. Still, in terms of gauging progress, you have to account for the margin of fluctuations that can be quite significant in some people.
I like to take weights every morning at the same time, and make note at the end of the week of your lowest reading. A combination of progressively declining weekly "lows" and improving body compostion via photos or mirror, and you're moving in the right direction!