It's set up such that each row represents the "state" of a tile. Either growing or fully grown, and either moving in a straight line or curving to one side. So in the case of growing in a straight line, the animation moves to the right and down one row until full growth.
If it needs to turn instead, you would use one of the other rows, but it's the same process. Basically the column position is shared across the entire vine, only changing rows as needed.
Hopefully that makes sense? 😅 It's made this way because that's how I was using it at the time, but you could also rearrange the frames in a way that suits your needs better.