I'm using a mac book air with OS X 10.9.1 and safari version 7.0.1. I did turn off autofill but nothing changed, Very curious behavior. I too looked to google for help but found none. Lots of problems with smart phones b;ut nothing helpful to me. Thanks for the reply.
Do you happen to have an Apple Store near by? If so, then my best suggestion would be to make a "Genius" appointment with them and ask them about it.
Since you were not using iCloud or some other syncing service, I can not see anything that might cause that result other than a bug of some sort. If you had a syncing service, then that could have been the culprit as they sometimes do strange things.
I just don't have a good answer other that maybe talk to Apple Support. The only other thing is maybe create a post about the issue on Apple's forums…that might be a better option as there are likely a LOT more Mac users on Apple's support forums, which would increase your chance that someone might know something.