The USSA has clarified that a ski marked by the manufacturer as 164 is not
Originally Posted by USSA
Q: If a woman's GS ski is marked, as 174 cm is it legal?
A: No, the ski must be marked with the proper length, 175 cm.
Q: If the women's GS ski is marked with 175 cm, but measures 174 cm, according to "unwound length", is it legal?
A: Yes, there is a +/- 1cm length tolerance for all skis.
Q: Does the tip deflector on slalom skis apply to the length of the ski?
A: Yes, if the manufacturer has marked the ski with the legal length, however, there is a special provision from the October, 2004 FIS meetings that require that athletes must finish with at least one tip deflector still in place, or be subject to disqualification.
(I also got the same answer from Jesse Hunt back in September or October)
I believe the World Cup example is different: those skis are specially made, so the graphic printed on the topsheet doesn't mean anything anyway.