IMO, this is a book ruling, only complicated by bizarre "Equity" consideratons.
A reasonable assumption is that this declarer (especially if he is an expert) has lost the place, presumably by miscounting trumps. Of course, when defenders dispute the claim, an expert declarer may well wake up to his mistake and try to correct his claim. In the normal course of events, however, a rational line of play results in one down.
The director might work out the correct line to make 12 tricks but as Gnasher hints, that allows a careless declarer to delegate his play problems, in the hope that the director is a competent player.
Also, Phil King points out that 2 declarers went down. If the director rules +2 (the best possible result for the offender, the worst possible result for his victims) then unsuccessful declarers will feel aggrieved - as will the defenders at this table!
A regular false-claimer might be frustrated and disappointed that defenders were sufficiently sceptical and vigilant to quibble about his original 13-trick claim
although he deserves no sympathy
If directors routinely give the benefit of the doubt to law-breakers, in cases like this, IMO, that transmits a disturbing message.