CSS22 don't understand box model

Hi, I don’t understand this section:


https://drafts.csswg.org/css2/box.html > 8.3.1 collapsing margins:

Two margins are adjoining <> if and only if:
- no line boxes, no clearance, no padding and no border separate them
- both belong to vertically-adjacent box edges, i.e. form one of the following pairs:
—— top margin of a box and top margin of its first in-flow child


Isn’t that contradictory?
I mean, if you have a top margin of a box and top margin of its first in-flow child,
then necessarily when you go from the top margin of the box to the top margin of the first in-flow child you cross the
(parent) box’s border box, as in:

parent box margin 
parent box border (this is always crossed, right?)
parent box padding
parent box content
child box top margin


So isn’t that contradictory with this statement?
- no line boxes, no clearance, no padding and no border separate them