fluctuated billing

Anyone of COX received their monthly bills, in which no changes have been made to services, add no change in service charges to phone calls:  BUT the received bill was (somehow)  app $10 to $11.00.--  or    8% to 9% lower than your usual bill.  **
AS YOUR FOLLOWING BILLS ONLY WENT BACK UP TO THEIR PREVIOUS AMOUNT, from past history.
Anyone received such result?
** I called Cox customer service, before my later bill went back up to its usual amount.  The answer I got was in CONTRADICTION, to the (present) current higher amount.
Going by what Cox Customer Service told Me, in my status as a Cox Customer, that lower monthly rater that I earlier paid --- shall be a rather consistent monthly rate.