Because this picture looks to be taken in the fall and it wasn't so cloudy to where there was hardly any sunlight
Also, they added grape vines since then, and the original picture was just grass, so it's not going to look exactly the same unless they get rid of the vineyard that planted them
The guy who originally took it swears he didn't modify it, but Microsoft did digitize it to create the popular background so who knows what they did to it after they bought the rights.
Also, they added grape vines since then, and the original picture was just grass, so it's not going to look exactly the same unless they get rid of the vineyard that planted them