www.theguardian.com/sport/2017/oct/27/us-teenager-who-won-golf-competition-denied-trophy-because-she-is-a-girl
So, a transgender woman can enter a female competition and win prizes (think cycling for example) but a real girl cannot enter a male competition, win and accept a prize.
(Yes, I know it's the rule of this competition and she's not complaining but it does highlight that with transgender women it's "heads I win, tails you lose".