A lot of digital scales struggle with repeatability (and accuracy). As made, you might step on and off several times in a row and get somewhat different results each time.

So the scale manufacturers came up with a cheat. The scale remembers the last measurement, and if the new one is close to that, it just reports the old one. That makes it appear to have wonderful repeatability, when in fact it doesn't. This also leads people to believe that the scale must be more accurate than it actually is.

But most of them only remember the last measurement, so there's an easy way to beat them. Weigh yourself, step off and pick up a moderately heavy object (a kg is plenty). Weigh again. Now put the object down and re-weigh. You'll likely see that your 1st and 3rd measurements, which should match, don't.

Of course, with a scale like that you aren't going to get accurate measurements just by eating 421 g of chicken. Or at any time, really.

TL;DR - Your scale is crap.