I noticed the other day that the Power Level of all of my Champions are dropping for no reason at all. Not making any changes to them in Artifacts, Skills or Masteries - and the Power Level is dropping. Not quickly or at an alarming rate but little bit at a time. Almost like there is a slow leak or drain in the game. I track all of my Champions on a spreadsheet (I am that anal) and that is how I noticed it. Why is this happening? 

The other thing that I noticed the other day is I put a 6 star artifact on a Champion - obviously with an artifact or accessory that has not been upgraded the power level of the Champion will drop with the new item. But what I saw as I upgraded the item higher the Champion Power Level continued to go backwards? How can an Artifact or accessory that is being upgraded to be "better" cause a Champion Power Level to drop? I thought the purpose of putting higher star items on a Champion and spending my silver to upgrade them was to improve the stats of the Champion. Maybe I do not understand this whole Power Level thing.

Please note - I am not that much of a noob - both the original artifact I had on the Champion and the new artifact were weapons of the Offense Group. The original Artifact was a base (silver) 5 star that I eventually fully upgraded to level 16. It 's primary stat was "Attack". The new artifact was a 6 star (silver) artifact with a primary stat of "Attack". I understand that putting a non-upgraded 6 star artifact on and taking the 5 start level 16 off - will cause the power level to drop. But as I was upgrading the new Artifact I noticed the Power Level continued to drop ??