Originally Posted by
Bob'sYourUncle
The EU mandates (re)screening all passengers to EU standards when they depart from a EU airport. So if you're coming from oustide the EU, you need to be re-screened if you're connecting, regardless of final destination.
Oops, of course it's all of the EU and not just schengen. Also explains the second screening in front of the B3X/5X gates in FRA for the UK departures.
Now if the bloody UK would just bite the bullet and join the schengen treaty..