Just as the ODST helmet is compatible with Mark VI armour, I guess other helmets are used elsewhere in the Halo universe and compatible with other armours.
Something bugs me about this, though. Why would Bungie randomly give an ODST arguably the most remarkable helmet in the game for no apparent reason? Furthermore, it didn't really seem to fit in the pictures. In pre-E3 pictures the Recon helmet looked photoshopped, as though it were added at the last minute. It looks as thought Bungie were trying to hide the identity of the character while at the same time drawing attention to her.