This may have been due to Beta testing going on almost right up to the release date.
The DVD would have needed to be prepared in advance so that it would be available to purchase from retailers on release day, so Props would have had to send something to be included on the DVD. I suspect that they decided to include the FSB and OSB on the disc to reduce the amount that would need to be downloaded, but as the program proper was still being finalised, they just used a download installer.
This would allow for the latest build to be available on day one.
I'm sure I read a thread here on the forum, where someone had asked Props if there was going to be a boxed copy that included the full program, and the answer was.....
Post #44 of this thread:
BTW, as someone in that thread mentions - If you get a point release (7.01) for a bug fix or something - then the version on the DVD becomes redundant anyway.