I suppouse that it is a way that different manufacturers make the cutouts for the windows. Also one shape could be stronger than the other. That is just my thoughts!! Do you rmember the Comet 1's they were totally square and were a cause in one of the breakups, along with the adf windows. Then they ended up being totally oval shape. And also have you seen the carvelle windows? It looks the shape of a rounded out Triangle!! That is certainly different!!
Safety is Everyones Responsibility.