Don't take things out of context. When Kakashi had that flashback, it was what Sasuke stated at the time. That was his dream for all of pt1 and alot of early pt2. He succeeded, or rather, Itachi let him by planning out the fight & letting him win. Kakashi was absolutely right.
Then, Sasuke found out the truth and got a new goal. To become hokage.
Kakashi met Sasuke when he was 12...he's not going to know what the kid wanted to do as an 8 yr old.
Sasuke's goals change throughout the series, he's not consistent in that regard. He changes his mind as he learns more.
8 yr old- " I wanna be a part of the Police Force & surpass Itachi"
12 yr old- "Revenge and I want to kill Itachi"
15 yr old- "Still want revenge and to kill Itachi, but I have to sever my bonds too"
16 yr old- "Itachi sacrificed himself & was in all of this pain to save Konoha and me? Gotta destroy Konoha"
17 yr old- "Nevermind. All other hokage are fails and the Uchiha are the best. I will become hokage."
BTW, when he said that he couldn't understand him, he meant in the sense that he never knows what Sasuke will do next and what he's planning. He cannot convince him to do otherwise, either. Also, atm he doesn't know why Sasuke really wants to become hokage and what he wants to do as one...hence the not understanding.
