You are right kacir that they haven't
always been released at the same time, but you are making a pretty pedantic point. The dates you listed were all pretty old, immaterial of whatever particular point you were replying to. The first few models apart, and one of them you quoted is just the announcement date (the Kindle Keyboard didn't go on sale until the end of August, an updated version then in May 2011), they have all been released around Autumn/Fall.
As previously mentioned in this thread, and posted elsewhere, since 2010 Amazon have been pretty consistent:
Kindle Keyboard August 2010/May 2011
Kindle 4/Touch September 2011
Kindle 5 September 2012
Kindle Paperwhite October 2012
Kindle Paperwhite (2013 Edition) September 2013
Fair enough, this is probably a pretty pedantic reply too....
Maybe Amazon will surprise us by releasing a new Kindle in the next month, I personally wouldn't bet on it, but who knows?