Probably comes from the fact that Oro explicitly stated that Itachi was stronger than he was, and that Oro is supposed to be the strongest of the legendary three.
When was it ever stated that Oro is the strongest of the Sennin? For all we know the Sennin can be the complete opposite of team 7 with Tsunade being the strongest and Oro being the weakest. Plus Frog Summon's>Snake Summon's.