Baby needs to be weighed on the same scale with the same amount of clothing (preferably naked) each time to get an accurate picture of weight gain. Different scales can give very different readings (I've personally seen a difference of a pound in two different scales); clothing or diapers can vary in weight and throw the numbers off. The scale should be zeroed before weighing, and baby should be centered on the scale tray. It's never a bad idea to do a second measurement (it should be close to the first) and then use an average of the two measurements. If your baby is very active or distressed, don't expect to get an accurate measurement. Babies grow in spurts rather than at a steady rate - to keep from needless worrying, it's generally best to weigh baby no more often than once a week.
And that is just talking about differences in scales made for babies. Hang in there!