I really don't understand this. Shouldn't vertical = top/bottom and horizontal = left/right? Why is the opposite?

Source:
https://github.com/flutter/flutter/blob/f5ddece56e0e9d21d3b3f2d46d96330cb7bd789b/packages/flutter/lib/src/painting/box_border.dart#LL340C5-L340C5

Could anyone help me understand if this is intended and why?
EdgeInsets work fine, every other symmetrical Flutter usage seems fine, but Border.symmetric doesn't make sense to me.
This was implemented/updated by @tvolkert about 3 years ago.