For your information / bug fixing
You need to provide more information.
Exactly what is the problem?
Looking closer at this, I assume you are expecting the surface area to be the height x width of the opening (glass blocks).
i.e. 2.0m x 2.45m = 4.45m (I am assuming your units are decimetres?).
It actually seems to be the surface area of the entire object - front, back, sides, recesses, etc.
In other words, how much paint would be needed to completely cover the object.
Strange that it does this for windows, but not other objects.
Objects don't get a surface area - or any other information come to that except for the classification and properties.
I am not sure if this would be classified as a bug or not.
But if you want it looked at, you should probably report it to your local Archicad support team and see what they say.