My iPhone has been on 5.1.1 because of the jailbreak, but we are a very small subset of all the iPhone users. Jailbreaking just isn't as popular as it once was (even though those that still do it are quite vocal).
I really don't see how the upgrade to 6.1 is any different from other upgrades - Apple made it pretty painless to do an OTA upgrade, so lots of people will do them. On top of that, the numbers are only for "tracked" devices. Not sure how Onswipe gets it's data, but if their sample set consists of above average technically savvy users, then that would skew the numbers as well.
Also, it would seem to me that the people who really care about the jailbreak would've stayed on 5.x until Sunday, as most have said in this thread --
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