Since airfields do not have a default hanger (H) boat entry spawn point, the first boat entry added to that field gets assigned to the H spawn button. In the above case, the first boat spawn from A145 is going to V140 (cursor in image) and is assigned to the H-button.
Map makers should put the initial boat spawn next to the airfield or vehicle base to serve as the H spawn so it is easier for the community to figure out which button goes with a boat spawn point.