I'll admit this is a bit of a geeky question, but I've always wondered why some aircraft have no windows in certain rows?

Row 1, 6 and 11 on the 777s for example have no window.
Row 6 on some 763's

Is it a structural issue or is there some logic as to why?

And is there a difference between no window and a window "plug" that seems to have been put there after the aircraft was built?