Random factor can be very interesting. I have Astaldos full sibling and she's a complete flop despite being WWW. Sometimes the match is kind, sometimes it's not :P
Stained horse glasses boosts a certain trait of the foal that's about to be born (int, scp, spd etc) You are to match weaknesses of one horse to another horses strengths in the hopes of getting a WWW but random factor plays a part in it so it doesn't always happen