http://support.apple.com/en-gb/HT202070
"If you turn off Background App Refresh, some background activity might still occur."
Background Refresh does not mean "no background". So, for a Facebook example, if you post a picture to Facebook, the upload process might keep running in background until photo uploaded.
And Notifications will wake an app. Change in location might wake things (ie. Suggested apps function, location based Reminders, alerts).
Refresh is "...lets apps check for new content and download updates".
Oh and: the battery usage number is total, not just background. The "Background Activity" label is a flag to alert you to there was background activity.
So, put it all together, an example from my phone: Podcast app accounted for 47% of the battery usage, with background activity, with background refresh off, due to it's a "music" app that keeps running in background when phone is locked and or using some other app on device, so one can hear audio.