Hmm. I just tried to reproduce this with old posts of my own and couldn't. I picked random phrases from five early 2006 blog posts that get basically no traffic and searched for them:
"I had been playing the accordion Davy lent to
Rosie during winter break"
"The language they're using is not that different
from the one I wrote PlayGUI to use"
"I've been playing a decent amount of music lately,
mostly guitar and piano."
"warm dry socks was the most important aspect of the
festival"
"This wouldn't be that bad, if it was not exactly what
happened a year and a half ago."
Google found all three. For each one there were either 2 or 3 results: first my old post, then one or two from rssing.com which seems to do something with my rss feed.
Trying them with Bing, it also found all five of my posts, and ranked them first in four cases. In the fifth case ("This wouldn't be that bad, if it was not exactly what happened a year and a half ago.") it ranked a goodhousekeeping.com post higher, which had all the individual words but none of the phrases.
(Disclosure: I work for Google, though not in search.)
Actually, rssing.com has nothing (directly) to do with your RSS feed. It seems to be a content scraper and mirroring/archiving "service," if I'm feeling charitable. And it looks like a site that wants to redirect users to its copy of users' content in order to get ad revenue if I'm feeling less-than-charitable.
In fairness to Google, that's always been a problem, and even in the good-old-days in which keyword-based searches were more effective, there were content aggregators that would copy the entire contents of phpBB-style bulletin boards (and USENET newsgroups) in order to rehost them and get clicks.
On the one hand, I want to say that it's precisely the sort of SEO/spammy practice that Google should be deprioritizing in search results. On the other hand, sometimes these copies/mirrors of content are the only extant copies of content when an original blog goes away. Although the motivations of the owners of these sorts of sites may not be as pure as that of archive.org, the result for the searcher is equivalent: the desired information is found even if it's only a rehosted copy.
Trying them with Bing, it also found all five of my posts, and ranked them first in four cases. In the fifth case ("This wouldn't be that bad, if it was not exactly what happened a year and a half ago.") it ranked a goodhousekeeping.com post higher, which had all the individual words but none of the phrases.
(Disclosure: I work for Google, though not in search.)