Flight routings

When I’ve searched for flights in the past, I have often been offered indirect routings and flew one 3 years ago. I wanted to fly BA from MAN to ABZ. The direct flight was on a Dash-8 both ways(boring) whereas for the same money I actually flew MAN-LHR-ABZ. Sounds mad I know but I got 4 sectors on 4 different 757s. On checking the other day; I was offered an even more indirect routing on BMI:
LHR-MAN-ABZ then on the return ABZ-MAN-GLA-LHR??? even for a sensible fare(@£140). Bizarre..