Pedigree Query

So I bred a quarter horse mare to Into Mischief. Into Mischief is heavily "inbred" to St-Simon one of the top sire of all times. St-Simon is over at the knees and strangely if you look at most of his produce I could not find any beside Into Mischief that is over at the knees ....When he entered breeding he was not regarded as becoming a superior sire .....I wonder what other St-Simon gene he is carrying that makes him such a superior sire . I look at his pedigree and am in awe of the number of great sires and producing mares....