Upon rereading a few earlier chapters of My Hero Academia, I noticed an oddity regarding Tomura Shigaraki's Quirk. Later in the story, it is revealed that whatever he touches with all five fingers decays uncontrollably until it's reduced to dust, with Tomura needing to be careful not to use his five fingers when manipulating objects he doesn't want to decay; much like Ochako's Quirk.
However, during the attack on the USJ, when Tomura used his power on professor Aizawa, only his elbow was affected, even though his entire body should have decayed. Is there an in-story explanation I missed for this discrepancy, or had Tomura's power not been cemented at that point?