I'm with you on that one dai. It baffles me too.
Wish there was an answer , because it isn't necessarily all about breeding ,
And as you say some will do Big quarry but not a little fox. A bad experience can knock em back ,but some will be born to do them ,just can't measure it by size or breeds in there.
I think if it's in them you can see from the 1st few.they just seem to want it and would pull off anything else for the chance of a go at the sharp end .
Shame you can't bottle it .