I has found the reason from Huang Mei Ying baidu tieba. What do you think of this?

[INFO] Tiffany sacrificed the lines she would've had for Yoona because she knew that Yoona didn't really sing in Bad Girl and this explain why Yoona had a lot of parts in The Boys. Yoona totally proved herself in this album, her vocals improved so much ♥
(c) sone united