Is there a discrepancy about Tomura's Quirk? 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?
As seen with different quirks as well, the user does not always have full control over it. Something which is also seen very well with Eri's quirk Rewind.

So much rather than assuming it's a discrepancy in the plot, the assumption that Shigaraki has no/clumsy control of his quirk is more likely.

Another factor at play might also be the speed at which the decay works. Through out the manga we have seen that the decay can set in almost instantly, but at occasion also act's slowly. Both of which might also be the case due to him not having proper control of his quirk.