The difference is very small, but think of it from the perspective of consistency and straightest string path, in relation to how the string lays next to the tie off string as it goes through the grommet. It's along the same idea as not crossing over the outside tie off strings on blocked holes.
The last cross weave will be over or under your last main string before it enters the last free grommet. (As an example, lets say it's under).
- As the string goes into the last grommet for tie off, the straightest path for the string would be for the string to lay next to the tie off string more towards the bottom.
- To keep the orientation of the string to stay laying towards the bottom, the knot should start the hitch loop starting under the tie off string.
- When you start your knot this way, your naturally tying the knot from under the frame to finish the pro knot.
Basically, the direction of knot loop, should match the last cross (over or under the previous main).