I think its likely that the roles they play are slightly different, as in Mansell gets right up the ground. At times this year we've had him coming in from the defensive side of the centre bounce. Thats not Cumbos gig. Thats not to say that we shouldn't have a player like Noah in the team, but both Mansell and Campbell play much higher up the ground than we see of Cumbo, which suits Coulthard or Green coming in for them IMO more than Noah, so it comes down to the role we expect them to play.
Take a look at these heat maps of when Mansell gets his disposals and its clear the role that he's playing.
AFL Fantasy stats by year, including player and team records going back as far as the 2012 season.
dfsaustralia.com
I think some of this is why we aren't as dangerous going inside 50 as we should be at the moment though, as we are deliberately trying to push up the ground to open up the F50 when we can. When teams don't necessarily follow us like melbourne didn't last week, they just left players back to double team it stunts out ability to score. Ie. we go higher up the ground and win more ball, aim to drag opponents up the ground to get our taller forwards 1 on 1. If this doesn't happen, we end up with more players around the ball and can win the ball easier up the field, but then have lower numbers than the opposition when we move inside 50, therefore those that leave players back can intercept at will.
If you look at Seths heat maps as well, last week he more resembled a winger than a forward.