[thstone]

My best guess is that by "staying" with the 265/40/18 tire size on the Base 987, Porsche could offer the 265/35/19 as a "sportier" optional upgrade on the 987 S.

If Porsche would have offered the 18" tire in a 35 profile it might have diminished the perceived upgrade value of the 35 profile 19" tire. In other words, the width stayed the same between the 18" and optional 19" tires, so Porsche had to make sure that the 19" had a lower profile.