maybe since x's story came before vii, yuna gave her gown to aerith. *nudge nudge*
Shit new headcanon
Tidus is Cloud's Great Grandfather and Yuna's FF X dress is a Strife family antique ( Tidus's last name isn't Strife, The FFX universe doesn't have them, it becomes the nickname of Tidus and Yuna's son, and then he adopts it into the family surnames when they come about ) Cloud's Mum is given this antique at her wedding with Cloud's dad, and when Cloud, Tifa, Aerith etc re- vist Nibelhiem Cloud heads into his old Family house and finds and dusts off the old antique that's Yuna's Summoner dress, during the Gold Saucer date off-screen he gives it to Aerith as a gift telling Aerith that it was his Mother's wedding present, and Aerith in return gives him a soft kiss.
Cait Sith pictured Aerith getting married in the dress during his prediction at the Temple Of The Ancients.
When Aerith dies, it goes with her however, as Cloud feels that it would not considerate to re-take the gift from her.