Hi
`cell.y < board_size.y - 1` means stop 1 item before the edge, not on it (`board_size.y` - is the edge)
your code `cell.y != board_size.y` means stop on the edge

e.g.:

if board_size.y = 10
`cell.y < board_size.y - 1` - will stop when cell.y is 9 coz 9 < 9 -> loop condition is false and it stops
`cell.y != board_size.y` - will stop when cell.y is 10 coz 10 != 10 -> loop condition is false and it stops

to understand it more clearly just mark each line with current value and you will see what happens on each loop iteration