Why does my data usage period change every month?

According to my bill, my period starts on the 6 of every month (e.g. September 6-October 5, August 6-September 5, etc).  Has been this way for as long as I can remember.

And accordingly, up until a couple months ago, my usage period would coincide with those dates too (so every month, my data meter would reset on the 6th of every month).  

Yet, for the past couple months (that I have noticed), that changed.  Last month, my usage meter reset on September 5 (instead of September 6).  And now again, this month, my usage period reset on October 4.

Is this normal now?  (and is next month reset going to happen on November 3?)