The problem with this is that the pair are not playing 4th according to the SC, West claimed they are playing 4th, and East did not lead 4th. The evidence, while not overwhelming, is that West gave MI.
Some of the answers seem to suggest that the fact East claimed he led a deceptively false card means there was no MI. I find the logical link strange, to put it mildly: it is perfectly possible he led a deceptive card
and that there was MI!
Some people seem to worry about whether we should assume mistaken explanation rather than mistaken play in the absence of compelling evidence otherwise. The answer is certainly yes, it is normal TD and AC practice. Why does the Law book not say so? No idea, but that is a question for the WBFLC, or possibly our 'Changing Laws & Regulations' forum, not this one.
Is the evidence compelling that there was no MI? Not really, in fact we are pretty sure there was MI. Until we know why West said '4th' and the card said '4th/low' we cannot be sure.
Do we routinely assume someone who says he false-carded was telling the truth? There are two answers to that. The first one is No, we do not automatically believe self-serving statements, we give them a certain weight in our consideration as to what happened when we come to make a judgement. The other answer is, of course, as discussed above, that even if we believe them it does not mean there was not MI.
The OP asked whether there was MI - Yes, it seems likely - and whether it is a weighted scores position. If there was MI then Yes, this is a typical weighted scores ruling. Only if the TD is pretty sure he knows what would have happened does he assign a single score. Weighted scores are the norm in adjustments.
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How are you ever going to allow a false card if you don't believe the player when he says "I deliberately false carded"
A bit extreme! How are we ever going to trust a girlfriend in anything she says if on one occasion she said "I do not want to kiss you." Of course the answer is that you make a judgement - in both the girlfriend case and the bridge case.
Another thing: East led the
♥2, and according to the OP,
East described their leads as 4th while knowing this to be both untrue and that he had led a card that was not 4th. I expect this was a typo in the OP, but if true then I think disciplinary action is called for against East.
Having considered all the evidence, I think that the evidence is in favour of MI. E/W need to describe their methods more fully if they are really are playing what they claimed, both on the SC and in answer to questions. So I am adjusting, and a weighted score looks obvious.
Suppose, for argument's sake, that the correct explanation is "We lead fourth, but occasionally a lower card when the fourth would be a high card: usually this is when we have six or seven". Now, does that mean that declarer would automatically duck the second round or hearts? Absolutely not. So in my view a reasonable result is:
40% of 3NT =
+ 60% of 3NT -1
If the explanation of 4th really was given by East then I am tacking on a 12 imp disciplinary penalty and reporting him to the authorities.
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If it's ok to use them, how can we arrive at 60/40 and not 70/30 or 90/10?
It is a judgement decision, like so many other decisions by TDs and ACs. We find all the facts, consider, consult, poll and make a judgement.