It would not surprise me one bit if Google did not test dynamic/server-gen pages further than standard, no offense to you, I am sure if Google does it, it does it for some reason likely related to influencing search results / spam or thereabouts.
I always take my old pages and leave them on my server but I re-code them to redirect the user AND the se's to the new page as follows:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//EN">
Then upload oldpage.htm and leave it in place, I got pages changed 3-4 years ago but I leave them because for some reason, certain se's / links have never updated and still from time to time a visitor is sent to oldpage.htm (which now kindly and immediately redirects the visitor to newpage.htm).
And, I think it's more user-friendly than a 404, there are SOME pages which I can not re-direct to the new page (because there IS no new page), some those I re-direct to the MAIN page, the few that are left get a custom 404 with several links to the main parts of my site.