I don't know what their ideas were but I have theories, of course!
P of Kildare was by Royal Charger, from a speed sire line but when he mated with Gainsborough Lass he must have nicked with her stamina, "St Simon" maybe, which allowed him to throw speed and stamina (Hi JInx).
On the other hand Knight's Romance, also by Royal Charger, inherited speed blood perhaps from his G/Granny Mumtaz Mahal.
Admiral's Luck's sire Precipitation is from a stamina sire line and when he met Gainsborough Lass, amongst other things, his Sainfoin blood nicked through Sierra with her (GL'S) Sundridge blood which is a legendary nick much prized by Tesio and other breeders of old; that particular meeting, maybe, eventually gave us Mainbrace.
This is just a sweeping, rough explanation. I don't completely understand it myself but it's a lot of fun.
You need some old books if you're really interested, about tb breeding last century. I've got 'Tesio, Master of Matings' by Ken MacLean amongst others by Ross du Bourg et al.. They tell you the history of our NZ sire lines- and the great mares. Go to second hand book shops- that's where I got mine.
It seems a long time ago but you can still see the relevance of those bloodlines. It's amazing when you can follow a line back to the 17th century even.
Good Luck.