It's a matter of accuracy vs. consistency. An inexpensive scale, like most bought for home or hotel room use, could be off by a few pounds in weighing an adult. I.e., it's not highly accurate. If you weigh 178 on your home scale Sunday and 181 on a hotel scale Tuesday, it's unclear what that means. Did you gain 3 pounds or is the difference due to the scales? They're not that accurate. But even inexpensive scales tend to be consistent; they give comparable results day after day. So if your home scale shows your weight increasing 3 pounds in 2 days, it's likely that's an actual weight gain rather than a measurement error.