Actually (might be stepping on Isom's toes here), but the NFHS changed that to the code RO for "Ride Out".

If a match were to have gone into double overtime and the top rode out the bottom, the score would be shown as 5-4 RO, according to NFHS.

I'm not too sure why you would have seen 6-4 tb unless they were in a round robin bracket and it was the third match and used to determine the tiebreak of a 1-1 bracket...

Hope this helps!