the reason most of u say Yuna doesn't look like Lenne is most likely because they aren't the same age.
Lenne is in her 20's and Yuna is 19. that could make Lenne look different from Yuna very much so.
but i think that if they are connected, that Yuna is Lenne's reincarnation or something like that. the same goes for Tidus and Shuyin. so there u go...that's what i think.