Hi kkkk1,

I believe you are using the engines A/T (autothrottle) turned ON, when this happens. If so, it's a normal condition and you are not doing something wrong and there is not a bug. Don't worry.

The wind vector (magnitude and direction) is constantly changing during almost all flight. So it will be necessary to adjust constantly the thrust levers to mantain the airplane's speed you chose.

Another factor that has influence is the variation of barometric pressure along the route, where small altitudes corrections are necessaries and can be viewed through the curved green line in the HSI (map mode) and in the climb/descend indicator.

In both cases, observe the burned fuel rate changes. The same happens in real airplanes.

Hope this help.

Pinatubo.