I have no idea but the level of details in this first icons and overall quality and coolness i guess individual approach was chosen for at least this default system icons. I doubt quality made "icon pack" for thousands of apps could be made over night and because of that i expect constant improvements in small steps for foreseeable future.
What happened to the new icon theme though? Canonical hired the guy behind Faenza and Faience. There was supposed to be a new icon theme for RR but I haven't heard tell of it since around November.