At night, or at work, my Android is usually offline. For me it's enough to only check email when I like to do so.
However, every time (or most times?) I switch on mobile data on wifi on my phone, it also seems to sync email (corporate Exchange account only, NOT the gmail app), and also checks for Play store updates for my apps. Same goes for Cyanogenmod updates.
Why does it do so? What's the use in turning off Sync, when apps can just ignore the setting and run background checks whenever they like?