Yeah, would expect that difference to be bigger but Vini is included here as has spent enough time at the club as a teenager. Otherwise only Carvajal is a starter while Nacho and Vazquez are barely playing.
Alba was released from the club when he was around 16 so doesn't fit that criteria. Pedri also doesn't qualify yet but will officially become a homegrown player after this season.
Also surprised Athletic only has 56%. I know Inigo Martinez is a Sociedad product while Raul Garcia and Berenguer are from Osasuna academy but would expect pretty much all others have grown up at the club. Didn't check that out but I guess some players must have been considered not good enough at some point and then moved between other (smaller) Basque teams at a young age before eventually making it back to their first team.