The most probable cause for this could be the cancellation of one or more of your ads. Depending on the ACoS of these cancelled campaigns your average ACoS is either increasing, decreasing or stays the same.

For example if these cancelled campaign have a high ACoS than your new average ACoS will decrease. If you cancelled campaign with low ACoS, which you probably won’t do, than your new average ACoS may be higher.

It’s a good idea if you cancel campaigns where you have lost money over a long period of time. Never cancel a campaign where you break even as the impressions will help other campaign make more sales.

Only cancel campaign where you loose money over a longer period and where you have done everything to make it profitable or at least to break even. For example you have adjusted the bids and paused certain keywords.