In one of the old books, maybe the Codex Imperialis or the Wargear manual from 2nd, it gave a pretty good explanation for the variation. Basically, flak was meant to protect from frag grenades and other light weapons, and carapace was meant for more protection. Flak was flexible, and carapace had stiff plating of some kind. The main part was that these designations referred more to the style or protection offered, but the appearance was dependent on where the armor was made. Carapace from high-tech worlds had layers of themo-plas or ceramite over mesh, whereas from a feudal world it might resemble a suit of knight's armor with thick metal plates.
Has anyone considered the possibility that what we've seen so far is in fact a brand new army and not IG?
I have a blog, check it out :P - http://forthegloryofgorkandmork.blogspot.co.uk/2013/04/tau-xv8-02-commander.html - just updated my blog 11/04/13
On that same note, anyone else notice that the camo scheme is totally new. I can't think of any current IG regiment that paints their tanks in a 2 tone blue camo. If I remember correctly the Bullygruns, or whatever the heck they are called, had similar colors on their shields. I wonder if GW is going to introduce a new regiment, or they are using the colors to make a distinction between human regiments (Cadian, Catachan, etc...) and ab-human auxiliaries.
On that same line, has anyone else noticed that this vehicle is painted up strangely? What guard regiment paints their vehicles in a two tone blue camo scheme? I wonder if this is the first sign of a new Guard regiment to be release with the book, or if GW is using different color schemes to differentiate between human regiments and ab-human auxiliaries.