The arrangement you mention gives half braking on the centre gear in the event of a full left or full right braking action turn, and full braking on the centre main in the event of full symmetrical braking, which seems reasonable. I suppose they could have split the centre main brakes left and right instead of front and rear. Perhaps there was some other consideration, leading them to keep the centre braking forces symmetrical around the gear leg?
Splitting the centre main brakes in this way makes for a very simple hydraulic arrangement. Anything else would require extra valves and possibly electronics to achieve the same purpose.