Very odd, everytime I've flown from MAN (usually to LHR) its always had 1-3 on the left and very hard to see usually because the 4-5 is 100+ more people. Never seen it the other way round, even on that gate. Agree the gate area is absolutely not wide enough but often it seems 1-3 don't even realise there is a seperate lane for them.
The priority security is also a 50:50 chance of actually being any faster than regular I find. In the end I think the only status thing I frequently use is the lounges (not as bad as everyone makes out unless I've been really lucky) and priority checkin when theres rarely a person in front of you.