During workout, there is no way to stabilize watts. For exemple, If i need to keep 350w during 3 minutes, I’m still moving around 340/360. After workout, if i check the curve, it’s not stable (up/down).
When checking some workout done by other rider, the curve is very very stable/flat, like around 349/350.
Others are using erg smoothing (normally Wahoo devices, but others such as Wattbike now have this) - They are not actually holding their watts that steady, that is just the way it is displayed to make it look nicer.
Like others have said, it’s probably just down to ERG smoothing on other devices. It’s really normal for power to fluctuate around 15w up/down. Don’t worry about it. If your did 10w up/down while not being in ERG mode, you’re actually really good at keeping steady power.