SharePoint adding, then removing Network Service to local groups

After upgrading our two server farm to SharePoint 2010, SP2, April 2014 CU, we've noticed activity in our event log when the Timer Service restarts daily and when FIM syncs run.  It looks like SharePoint is adding the Network Service account to the local groups IIS_IUSRS and WSS_WPG, then removing it a little later on.

Does anyone know why this happens?  Is it expected activity or a bug with the version we are running?

Thanks in advance for your time- Rob