Mass Combat: Failing Strategy Roll with a bigger Margin of Success than the Winner

[Pseudonym]

So weird conundrum.
Basic set and lite say:

Each competitor attempts his success roll. If one succeeds and the other fails, the winner is obvious.

Fine, so let's say:
Sean has Strategy-20
Anna has Strategy-15
Sean Rolls a 17; Anna rolls a 15.
Anna succeeded because "The winner is ***obvious.*** (Is it?)"
But her Margin of Victory is... -3?
Where does that fit on the Mass Combat's Combat Results table (p.47)?

Do Quick Contests have a minimum Margin of Victory? Should a margin of victory be floored at 1? Also, can I get a generalized ruling for all Quick Contests?