The natural expansions have a different amount of airspace near them leading to issues when trying to defend flying units.
Shown here is the top right position (it works on the bottom left position as well). Even a marine will hit a flying unit at the farthest possible edge of the map.
Now compare this to:
Where even thors cant hit the flying unit.
This is a pretty serious imbalance imo. I first noticed this while trying to defend muta harass and had never really run into it before. Why does blizzard insist on not fixing these things