I'm not sure what you guys are questioning about it... Luke is coming directly out of the side hatch of the AT-AT. So the AT-AT's main body is straight lined up to the end of that platform. Perhaps it is awkwardly positioned against the tree for support... though it seems incredibly lazy not to figure out another way.

And for the platform and the immediate area to match up you could either remove the trees you see in Luke & Vader's scene or you could add in all those trees in the other shots but then it would probably cover up much of the lower half of the platform. probably the reason for the discrepancy in the first place.

I would assume the Empire wouldn't leave behind huge trees to get in the way of AT-ATs docking to the platforms. They cleared that space when they put that thing in.