Transformation Arrow direction - Merge doubles

I replayed Lecture 22 by myself and noticed the following:
In the video when the remove doubles action is executed the vertices snap together to the vertices closest to the green arrow.

With my replay of the actions my arrows are facing other way and when i snap them together it looks like the original object.

I know i can move the vertices out again to replicate the object in the lecture but i realize that I don’t really understand the concept of the positive vs negative numbering with the arrows and can see how this will slow me down when modelling objects. So my questions are:

1.) Can the arrows be flipped around? (Positive to negative and vice versa)
2.) What is the importance of positive numbers vs negative?
3.) Is there a upcoming lecture this that will explain all of this?