That's a very interesting observation. I've not seen anything official about this (and we're not likely to, either) but I've had a similar feeling. It's hard to say anything definitive without a ton of data, but something like fixed "test positions" seems to have rolled over the past year, first resulting in the Position #6 Bug [webmasterworld.com] a year ago.
My feeling (not solid analysis yet) is that the Yo-Yo Effect [webmasterworld.com] may also tie in, because it certainly seems to rely on certain forced positions. The fact that your urls were just recently released from the -950 penalty may be an important part of the picture - as a guess, it could be a way to test whether they perform well against the current competition.
We'll see if anyone else has similar observations.