Factual error: At the beginning of this episode, Pompey and Cato talk about the battle against 'the Illyrian pirates'. There were no 'Illyrian pirates' in Pompey's days. Those pirates had already been wiped out during the Illyrian Wars of 229 and 219 BC. The pirates Pompey did fight were from Cilicia, in the south of present-day Turkey.
Factual error: In this episode they make comments on how traitors will go to Pluto, meaning it in the sense that they will end up in hell. Pluto was not equal to hell; everyone who died went to Pluto.
Answer: Its Pullo's nature to resent people of higher standing than him. His jealousy of Vorenus in later episodes, which in part leads to their falling out, is proof of that. From Pullo's point of view, Vorenus just got better breaks to get where he is in the army and so, isn't really his superior. That, plus his natural arrogance is explantion enough for his behaviour in spite of the fact that he broke the rules and was being fairly punished.
roboc