I agree with what has been said here already. Use one scale and weigh yourself at the same time every week. I weigh in the morning after the bathroom, but before breakfast.
I've heard that digital scales can be unreliable for a variety of a reasons. If it is a cheap one or old, then it might be the scale itself. Digital scales are also thrown off by an uneven surface beneath. If yours is on carpet, make sure you move it to a flat surface. Also, slight dips or slopes in the floor and give you wonky readings. I leave mine in the same spot on a tiled floor.
However, if your scale is consistently 7 pounds different than the YMCA scale, then just account for those 7 pounds and subtract them from your weight.
If you are in the market for a new scale, then I have a recommendation. I did some research and got the
Taylor Lithium Electronic Scale from Target. It was only $25-$30 and it has been extremely accurate for me. I've tested it out and re-weighed myself several times in a row, and it always gives me the same result. I've been very happy with it and would rec it to anyone looking for a good digital scale at a low price.