Okay, i get it.

The peeps of Kilika would have turned into monsters if they were not sent. They did not accept death AT ALL!

People like Tidus's mother must have accepted death and did not have a strong bond in the world, so that is why she went to the Fareplan, because if she still had a strong bond to the living world, she would be like Auron, an unsent.

Auron and Seymour....Okay, i get it now, they are unsents because they still have strong bonds in the world. Seymour relized his plans for Spira's "liberation" sucked so he just accepted it and boom, he went away for good.

Tidus was a dream of the fayth in a way, he was neither alive nor dead...

Okay, one more question, If people like Auron are Unsents, because they still have strong bonds to the living world, then why didn't Tidus's mother stay on the earth to care for her son? I mean, in Tidus's flashbacks, the mother seemed to care for him a lot while Jecht was waltzing around town acting like an idiot in a way.
So...Did Tidus's mom quit loving him when she died or something? Because you would think she'd still have a strong bond to the guy.