Forum Moderators: DixonJones
See the google blog for a recent announcement about how they have recently changed length of visit measurement. They say they've now reverted to the "original" method --- but they gave no previous announcement, to the best of my knowledge, about changing it to the experimental method in the first place. At any rate, Google Analytics recently recalculated all your length of visit stats going back to whenever the experimental method started.
What you're seeing is probably the result of the reversion to the "original" method.
I thought it was pretty high-handed of Google Analytics to completely change the way they measure something, twice, giving users minimal information about the changes. No announcements within the product for the change-back that happened this week, for example. You have to keep watching the blog, which is notoriously unreliable. One thing I want from them right now is the date of the original change so I know how far back the recalcs will go.