Board Thread:Naruto Discussions/@comment-34436445-20180703141210/@comment-1512913-20180705124641

Maybe Sasuke explained it off-screen?
 * Naruto/Sakura/Whoever: 'Sasuke, about your Rinnegan... it looks somewhat different'
 * Sasuke: 'What do you mean?'
 * Whoever: 'It has six tomoe in it... it kind of looks like the Sharingan, ya know?'
 * Sasuke: 'That's because that is Sharingan dumbass'

Pretty much.