When a Tyrant is exposed to biologicol agents (poison radiation etc.) before or during the time that they are infected by the T-virus, it goes through rapid mutation and changes the attack power of the Tyrant. The only T-267 reported was wearing patches of USS body armor. The reason for this is guessed at but Umbrella figures that sticky residue caught the armor and kept it there. As in case of HUNK's captain in Resident Evil: Premonition, his claws drip poison into the blood stream of the target and kills them before they turn into a Poison Zombie. Umbrella didn't continue this line of B.O.W. because it would lead to more outbreaks due to its continued mutation.