The most accurate way to use a bathroom scale is to put it on a level and solid surface so it will not rock, zero it out then weigh yourself, then yourself holding the wheels and subtract the difference.
Repeat this at least one more time, if off repeat a third or fourth time until you get a stable reading, if that does not happen then average all the results.
This is not the absolute way to be sure of the real weight but the best we might have in some instances.
If weighed by just placing on a scale, not very accurate as scales are less accurate at the extremes of their range, it could be quite far off.
My qualifications, so you know I have a bit of an idea about all this:
1) Internet business owner, retired, personally used the method above thousands of times before we had a commercial shipping scale and for smaller packages, one sitting on my desk I still use all the time, certified digital postal scale.
2) Verified the accuracy on hundreds of packages using the bathroom scale method, it was always very close to actual shipping weight.
3) Thousands of packages shipped were weighed by the shipping companies and corrected if not right and I was billed for it so learned to get it right.
4) Prior stage of my life I was a federally a certified Weigh Master working for Burlington Northern Railroad and weighed box cars with very strict tolerance scales and methods.
5) Just as a side note: I built the lightest full suspension mountain bike known in the world 15 or so years ago, I used a gram scale on many parts, 20.4lbs total and very rideable race built cross country bike I rode hard and did not break. I have been making cars lighter for over 40 years as well.
Thanks for weighing them the best way you knew how, much appreciated
Rick