I am not very good at interpreting rules but does this rule mean a robot has a max height limit?
No. There is infinite vertical expansion as long as you are inside the expansion zone. The expansion zone is denoted by the U formed by the white tape. When you are inside that zone, you may expand as far high as you please in order to either score a cap or launch a ball. However, when you are outside that white U, you must not have a height greater than 18". There is a maximum limit on horizontal dimensions (maximum of 36", regardless of being inside or outside of the expansion zone).
So… one of the starting tiles is in the expansion zone and one isn’t.
Does that mean in autonomous you can expand if you’re on the tile that is in the expansion zone?
Note: a robot does not have to be fully within the expansion zone to expand. As long as it is touching one of the expansion zone’s tiles or the white tape surrounding it, it is completely legal for the robot to expand vertically.