For a scale intended for weighing people, are small test weights such as 3-pound dumbells and pint cans of food really any help? If the scale’s wonky, it doesn’t seem like you can rely on the result for a pint can, either by itself or with you on the scale holding it. Sure you might see the 2.2 pound difference if you stand on the scale with or without a liter container of water, but that doesn’t mean both results aren’t five or ten pounds wrong either way.
Suggestion: Has anyone else in your house recently been to the doctor, or otherwise had a chance to be weighed on an accurate scale? If so, weigh them, and then extrapolate.