For my 10 pennies worth its just math, as genetics is just math, especially with hybrids. Genetics is only predictable on the grand scale and the more variables you add the grander the scale needs to be to get a portion of predictability. I had a bull/grey X collie/grey was a thick coward with no pace and no stamina but hyperactive at home. He had 3 separate gene pools to throw a variety of traits, sometimes they will be the right ones sometimes not, in his case not. The Bull X greyhound sire was a decent dog if a little slow, the collie/grey dam was a sharp ferreting/lamper bright as a button