OK, so you'll need more than a Japanese 3DS system and a copy of Sega 3D Fukkoku Archives 2 to take advantage of the "reason" mentioned in the header above.
Specifically, you'll need to own a copy of the first Sega 3D Fukkoku Archives, too.
Why? Because people who own--and have played, for at least a second or two--both Sega 3D Fukkoku Archives 3DS games will gain access to the Sega Mark III (aka Master System) version of Fantasy Zone that's been updated with FM sound.
If that last bit means nothing to you, read this post on segaretro.org. Also, watch the video above to see (and hear) this iteration of the original Fantasy Zone in action.
So, with that, now features 10 old Sega games, including Altered Beast, Fantasy Zone (Sega Mark III port), Fantasy Zone 2 (Sega Mark III), Fantasy Zone 2 DX, Galaxy Force II, Maze Walker (Sega Mark III), Power Drift, Puyo Puyo Tsu, Sonic the Hedgehog and Thunder Blade.
You've got a few options if you want to play this version of Sega's pastel-tinged shmup, by the way. If you already own the first Sega 3D Fukkoku Archives, you can simply buy the second compilation, which is due out on Dec. 23. And if you don't own either title, you can pick up the "double pack" that will be released on the same day.
(Via famitsu.com and neogaf.com)