Android custom renderer: OnMeasure only receives "MeasureSpecMode.Exactly" ?

Hi,
i created a custom renderer, and added my control to a xaml page.
I overrided OnMeasure to make the width proportional by a factor to the height.
I put the control in a Frame and set HorizontalOption and VerticalOption to Center and Width to a fixed value.

But in the android custom renderer, OnMeasure never receives MeasureSpecMode.AtMost, only MeasureSpecMode.Exaclty

Any idea what's going on ?