I suspect you were on a standard fare (something like NOWBMI basis) on the return which only earns 300 miles i.e. 600 x 0.5, and a tiny fare on the outbound e.g.(NBABY fare basis) which only earns 150 miles, i.e. 600 x 0.25.

You have to pay quite a lot of money to earn 600 miles from MAN-LHR these days.