It looks like it's picking the areas of least content that would be the easiest to stretch. Only thing I can suggest, is to do it in small chunks by using the marquee tool to select the area you want to stretch.
Content Aware Scale will always stretch area more that have less detail. That's just the way it works. There is much more detail on the left so it gets stretched less.
This photo is not the best candidate for Content Aware Scale. You will probably need to use a variety of techniques to make it look natural.
Select and content aware scale the right lower side.
Fill the upper right with a gradient that matches the sky color.
Select where the clouds are being chopped off and Edit > Fill > Content Aware.
Select grass on left and Edit > Fill > Content Aware.
Select sky area on left and Edit > Fill > Content Aware.
Assuming that the stone wall continues to the left, select a good piece of it and duplicate it to fill in the area.
You can use the patch tool on the wall to touch up any areas that look too repetitious.
Fill in the blue sky with matching color.
Select the left edge of the tree and use Content Aware Fill to round the left side.
Thanks Chuck for the explanation
Thanks Barbara for suggestions