As this is slightly on topic here I thought I would ask.
If you reduce a 10 to a 9 do you gain 1 point? The table in the PHB seems to imply that with a -1 but I can see no clear text to back that up.
The other reading of that would say that as the table does not show anything lower than a 9...
I clearly must be reading this wrong. I read this as saying that the second time the fighter tried to put a mark on an enemy the first mark would go away. Thus the "supersedes" text.
I can see how perhaps this was just trying to say that the fighter's mark could be overwritten by some other...