Aude, in young children (and I guess that's what both speakers were asked to assess, since this is usually requirements asked of very young children who have at best only limited training), muscle quality may not equate to muscle definition.
Things like long Achiles tendons is usually seen as a 'natural' (something you get without training) quality that will be favoured at audition. It's assessed usually by asking the student to bend the knees as much as possible without lifting the heels off the floor. The deeper the plié usually means the longer the tendon.
This is only one example of things that will be favoured by those schools. I'm sure there are a multitude of other exercises that will be asked of the young dancer at such audition (believe it or not, I did that type of assessment as a teenager, but I can't remember at all what we had to do specifically on the day -must be the nerves!)