Not all of the data was specific to HTML emails or attachments, but to apps running in the background, most especially for iOS. iOS, applications continue to use a data connection for up to 10mins - this is by the iOS design. Then their cut-off or placed into a dormant state saving updated data to storage ... ONLY iOS core apps or Apple apps along with any third party app using VOIP continues to use data in the background.
Now consider how Jobs told the world that when you place an app in the background not to worry it'll continue where you left off, its that easy. What he nor Apple stated was the continued data so consider:
Podcasts you subscribe to get updates (audio and video; that you've allowed to use cellular data) get to download MB/GB of data ...
Maps - a HUGE culprit for this!!!
Nike+ made by Apple so it continues.
iTunes U - LOTS of data on subscribed university content (very excellent BTW, I'm still trying to find a way to migrate this to my BB10 device).
I'm not sure what you mean by synching email headers? I'm thinking the title of an email is a header or the top half of a bounce-back. I'll need a reference/example to understand as I haven't heard that term in a long while.