The decision appears surprising. The rationale for it was that Yves Saint Laurent had failed to prove that YSL was a well-known mark for fashion goods, and that a distinction was to be drawn between Yves Saint Laurent, the person (who was well known) and the YSL monogram.
The Court also considered that the applicable case law showed that there would not usually be a likelihood of confusion between acronyms and short (i.e. three letters or fewer) marks. The courts attribute heightened powers of perception to consumers confronted with these types of mark so that the differences between them are exaggerated. There is also a policy reason that short signs should not be granted monopoly powers of protection. In this case, the addition of the words "Skinny Love" beside the SL removed any likelihood of confusion.
Yves Saint Laurent may well feel hard done by in this decision as there does appear to be an argument that consumers will associate the Skinny Love sign with the YSL mark.