In the transit area you mingle with people who come from countries which the uk (and other eu countries) do not consider security to be high enough, so you get checked again. In lhr only domestic arrivals are kept separate from other arrivals till they are cleared.

FRA shows it better. If you land in Z they csn keep clean and unclean apart so you may or may not reclear security depending where you land from. If you land in B they cannot so you get security no matter where you land from