>...but it might mean that different revisions of the
> same game have the same or similar encryption key.

that is exactly what it means. I believe to remember quite well one of the first things we found out when cps2shock started releasing Excel tables of parts of the unencrypted Street Fighter Alpha roms (long before the time of XOR's).
And that was that that different revisions boards of the same game use the same encryption. It's described in Report 2 on aforementioned page.