Okay...just re-read the Success and Failure box and,as you point out, it is me misunderstanding or misreading the rules. My character and the opposing squire had both hit each other a couple of times before and our respective HPs were close to the 'Unconscious' threshold. Then in the last attack we both 'succeeded' and so I assumed of the choices in 105 I had to choose 'go to 108' as we had both succeeded...but it should have actually been a 'Win' or a 'Partial Success' (I can't remember which of us had the better roll). That makes a lot more sense...though I guess it is still possible for both characters to knock each other out, if it had been a tie and they had both been close to the Unconscious threshold as they would both have taken damage....but as you say, the chances are fairly small.
Yeah...I meant to say half my HP not a quarter, which I inferred from the opponent's threshold.