You need to look at why the numbers might be different (even down). For instance, how is defensive rating calculated? Is it based on opposing pg production, and if so wouldn't the fact that he frequently guards opposing SGs affect this? His shooting percentage is down, but that's not because he's shooting worse, it's because he takes more jumpers than he used to and even good shooters make fewer jump shots than layups.
As for his assists and turnovers, the numbers are only slightly off from last year (earlier this year they were slightly up from last year) so that's merely fluctuations that all players see in their stats. Any time Rondo has a stat that drops from 1 year to the next people love to call that regression, I guess that's based on the (ridiculous) assumption that most stats for other players either stay the same or go up every season.
You are missing the point of the original post (which, by the way, I wrote in another thread, and for some reason was copied and pasted by another poster as a new topic ...)
It's not one number going down, it is almost all of his stats slumping. And you are right, there are fluctuations in stats of all great players, even Lebron.
The thing is that almost ALL of Rondo's numbers are down this year, even taking into account his hot start. There is obviously still time for him to turn it around, and I hope that he does. But that does not change the fact that his play over the past 15-20 games has been so abysmal that it has dropped his numbers down to career worsts in many categories, even including the fact that the first 10-20 games of the year he was putting up career highs.