Pelipper got +10 BST in Gen 6 and Swellow didn't, which is a missing link, but then in Gen 7, along with Pelipper getting another ability, Swellow got +25 BST. The +25 to Swellow can be seen as Game Freak making up for the +10 BST given to Pelipper, but then overcompensating. That means that, in the pair, in the net change of BSTs was +15 to Swellow compared to Pelipper, so Pelipper needs +15 BST to make up for it