Kg to stone

Did you do those weigh-ins one immediately after the other, no activity in between, no eating/drinking, wearing the same clothes/shoes, no visit to the bathroom in between? If all of that is true, and all of it was on the same scale, then maybe iffy scale.

If all of that list of things were not true, like it was different times of day and you'd done things in between and/or were dressed differently . . . then you just weighed a different amount at the different times.

Our bodies wander around up and down a few pounds over the course of a normal day, even without considering the clothing differences. It's mostly because of shifts in retained water - we just drank some, we urinated, we sweated some out, we exhaled moisture, whatever; or changes in temporary contents of the digestive tract, basically things that are on their way to becoming waste. When you drink a cup of water, or eat an apple, it weighs just as much inside your body as it did in the cup or in your hand, respectively. Most of that weight ends up in the toilet eventually, if I may be graphic. Only the calories in the food have the potential to affect your body fat.

If it was a different scale, well, they were calibrated differently, and there's no way to know which is right. It doesn't matter, really. Pick one scale and use it to gauge your progress. Ideally, weigh yourself at the same time of day - first thing in the morning after bathroom, before eating/drinking is good. Also, weigh yourself in the same state of (un)dress every time.

Then look at the trend in your weight over 4-6 weeks, whole menstrual cycles if you're a woman of that age. Shorter term fluctuations are more about water and waste than about body fat levels. Body fat is the part we're really trying to lose, right? That happens a little bit daily, plays peek-a-boo on the scale with the bigger water/waste shifts, but shows up in that multi-week weight trend.