User blog comment:IamJakuhoRaikoben/Chapter 499: Gray and Juvia, Review./@comment-4945830-20160831204621/@comment-7418318-20160831231232

That's what I've been trying to tell you... they didn't have to be "really lucky".

Purely statistically the chances of one person being a siutable doner to another is over 50% for the type and exactly 50% for the Rhesus Factor (if we assume an equal spread of types among the Ishgar population).

Really lucky was Lucy's 1 in a billion (actually 1 billion times the number of people invading cube, so more like 1 in 50 billion) chance of escaping Alegria. Compared to that beating two close to 50% chances seems like a sure bet in comparison.