It could be a mistake on Porsche's part. I have seen them make a mistake on the off-road tire as well, quoting it as 255/55 when the fact is they have only approved the 235/60. Sometimes a copy editor feels compelled to make content corrections when they are only to check for grammer. The 255/55 is so otherwise common for the Cayenne in the 18" wheel, I'll bet a copy editor corrected it.