I am a grid novice too so take this with a grain of salt. I don't think there is a bug, because even though you have a 4x4 grid when you count the lines startin

I am a grid novice too so take this with a grain of salt. I don't think there is a bug, because even though you have a 4x4 grid when you count the lines starting at 1 you can count up to 5 including the last one. A little counterintuitive .. scroll down to the labeled grid right below the demo and it might make more sense.