All I can think of is that he is third choice center behind Seedsy and Jackson. Seeds has been injured for a bit of the season as had Jackson, and sometimes Jacko has played in the second row leaving a spot available, hence why Derry has got quite a few games this season.
I'd say he's still just behind Seedsy and Jacko in the pecking order for this season, but it's good to know we have a more than able replacement should one of them get injured