I can't comment on LGW but the 'logic' [hohoho] at T5 is that outbound to GVA you have passed through the high standards of UK security at MAN and can proceed onto the GVA flight through flight connections knowing you are safe and secure. On the return you have only been screened to mickey mouse jonny foreigner swiss standards and could pose a threat to everyone. Therefore you have to be screened again to the high standards we expect in the UK and jonny foreigners can only dream of.
And, illogical though it may be, I can almost accept this. It's the LGW vs LHR differences I really struggle with.