When you're confined to one seat for up to 14 hours, there are a lot of things you start to notice, like those tiny hole at the bottom of airplane windows or the weird triangles on the wings…
The answer is, that each airline configures the plane differently.
Plane manufacturers actually plan for seat row positioning which generally line up with windows. However, all that changes when an airline buys a plane and wants to increase its profit and competitiveness by cramming more seats into the original design.
As you may know from all the times you've purchased a "window seat" only to find a stretch of plastic wall centered with your seat, airlines rarely go with the manufacturer's suggested layout. Obviously, this is likely due to moving the rows closer together (reducing legroom) in an effort to cram as many of us on board as possible. It all makes sense now, right? Sigh.
Manufacturers like Boeing may recommend a layout of 3+3+3 on a Boeing 777 with a 32-inch (81.2 cm) pitch for economy passengers. The advice is often disregarded by airlines and this changes the relationship between seat and window.