Question re. smart guides
 

There's some details on alignment smart guides I've never quite understood. I can't seem to find an explanation in the help documentation or pdf manual.

When nudging a horizontal line down to make a "T" intersection on to vertical arrow, guides in the following order:
>first - 2 up and 2 down arrows, pointing towards each other
>next - a blue line through the horizontal line, only extending a little beyond the edges of the line.
>last - a magenta line that extends through the horizontal line across the entire page. (This might coincidentally be the center of the page - so probably an artifact?)

All seem to put the horizontal line at the very tip of the vertical one. What's the diff?