Its not Halo 4 thats ''undertracked'' but Halo 3. Halo 4 online numbers have been very poor for a 10million seller.
Its Halo 3 that hasn't been updated for a while. It must be on 13-14 Million copies by now, but VGC still has it at 11.9.
Remember, old(ish) 360 have not been tracked properly by Vgchartz.