All the left and right shift/ctrl/alt keys and Fn and Win keys usually have individual lines. You can do two key rollover on a plain switch matrix, but if you have three keys down, with two in the same row, and two in the same column, unless each switch in the matrix has its own series diode (not practical for membrane matrix keyboards) you get a phantom key-press at the intersection of the row and column that only have one key pressed out of the set of three. Thus if the user is going to press multiple shift keys, they cant be part of the main matrix.