By dual pixel they mean a horizontal line that is one pixel tall and two pixel wide. Each cell's side is 7 such dual pixels arranged diagonally. But since you stack tiles so that the grid line overlaps, it means that two 7-wide cells are together only 13-wide (and not 14). The top dual pixel of one is the bottom dual pixel of the other.
Going smaller, two 4-wide cells will again lose one line to the overlap and thus join to make a 7 (that's how 7 divides by 2).
Three 3-wide cells join and lose two lines to overlap and thus again join to make 33-2 = 7
Six 2-wide cells join and lose fives lines to overlap 62-5 = 7.
Or in general for n cells, there are n-1 overlaps, so if they are k-wide the combined cell will be n*k - (n-1) wide.
Going smaller, two 4-wide cells will again lose one line to the overlap and thus join to make a 7 (that's how 7 divides by 2).
Three 3-wide cells join and lose two lines to overlap and thus again join to make 33-2 = 7
Six 2-wide cells join and lose fives lines to overlap 62-5 = 7.
Or in general for n cells, there are n-1 overlaps, so if they are k-wide the combined cell will be n*k - (n-1) wide.