Roll center question

I was looking over Carroll SMith's Tune to Win and he says that the front roll center will always be lower then the rear. I am confused by this because with an unequal and non-parallel link suspension the roll center can be placed where ever. So why is roll center in the front always lower then the rear? And if the roll center is at the same height for both front and rear what happens?

If the roll center is higher in the front that will increase the tendency to understeer. If it's equal, that will probably increase the tendency to lose traction suddenly near the limit vs. losing it earlier but being more controllable.