Why does the area around the runway change textures when the landclass around the area is changed?
Because that Airport background poly is designed to change color from tan to green DEPENDED ON THE LANDCLASS TILES AROUND IT.
FSX has two types of landclass tiles: WET, and DRY.
Wet landclass tiles give a Green airport background color.
Dry landclass tiles give a tan airport background color.
In situations where both dry and wet -type landclass tiles exist around the runway, you will see a variegated pattern based on what would be the landclass boundaries if the airport background poly was not there.
FSX by default uses what FSX_KML calls the following tag:
Airport_Backgrounds_MaskClassMap_Flatten_ExcludeAutogen
The "maskclassmap" means that the landclass tile will be overridden by the airport poly texture, and that poly's color will be based on the landclass tiles below it. Wet or dry. Green or tan.
That's how they did it.
If you want a green background, you will have to change the background landclass tiles to wet-category tiles. Otherwise it will be brown/tan no matter what.
An alternative is to go with no background poly texture. You can acheive this with the tag Airport_Backgrounds_Flatten_ExcludeAutogen in FSX_KML.