Comparing 986 and 987 tire sizes

[WhipE350]

I was comparing tire sizes from my 986 'S' to my 987 Base. The wheels are exactly the same 8/57 front and 9/43 rear. What I don't understand is why the 987 has 265/40-18 on the rear. Was it for looks, ride comfort? It seems strange because there is a 3.7% difference from the Base's rear to front when if they had used a 265/35/18 there would only have been a -0.4 difference. I'm not an expert at any of this just a curious owner so maybe it doesn't matter at all, just found it interesting. It does seem to tell me the wheels w/tires are completely interchangeable between the two vehicles. Here are the numbers I worked from:

986 'S' -

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
225/40-18 3.5in 12.5in 25.1in 78.8in 804 0.0%
265/35-18 3.7in 12.7in 25.3in 79.5in 797 0.9%

987 'Base' -

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
235/40-18 3.7in 12.7in 25.4in 79.8in 794 0.0%
265/40-18 4.2in 13.2in 26.3in 82.8in 765 3.7%

987 'Base' with 35s in back (note this is the closest in Diameter)

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
235/40-18 3.7in 12.7in 25.4in 79.8in 794 0.0%
265/35-18 3.7in 12.7in 25.3in 79.5in 797 -0.4%