It is probably just a supply issue for the 987, as they are different wheels.

Here is the excerpt from the Porsche AG press release about the
2007 Boxster:

"For the first time, the Boxster and Boxster S are available with the 19-inch turbo
forged alloy two-tone wheels that were developed exclusively for the new 911 Turbo.
For Boxster models they are offered as an option in sizes 8 J x 19 (front) and 9 1/2 J x 19 (rear)."