The groove might have been for some type of link or indexing, but the only cannon rounds I have seen 3omm and larger with link grooves are the 35mm. I have never seen a GAU-8 or GAU-9 case with a groove like that before, and it is much wider than a typical link groove.
As to why the Apache TP is in a GAU case, it is a marketing thing. The "salesman" had no knowledge of the proper case/projo combination and no inclination to learn, but the hole and projo were close, so why not force them together?
As far as why there is no APDS projo for the Apache, it is a much slower round than the 25mm or 30mm GAU rounds, so APDS doesn't work as efficiently as HEDP shaped charge rounds. The DP stands for dual purpose which means armor piercing plus high explosive/frag. The Apache wasn't designed as an antitank specific machine as much as the A-10 Warthog. The Apache is set up as an overall battlefield flying artillery piece, with Hellfire missiles and 2.75 inch antiarmor submunition rockets. The 2.75 inch rockets also include flares for battlefield illum, WP-smoke for screening and incendiary, and a flechette round for antihelicopter use, besides the 3 sizes of HE rockets. Did I mention that it also carries stingers for air-to-air defense?
The 30mm gun is for destroying lighter skinned items that the pilot doesn't want to waste a rocket or missile on.