It has to do with their genetics. I'm new to Leos also but have been reading allot. Best to read the stickies at top about the genetics or Google Leopard gecko genetics. There is a ton to learn before attempting to breed them and as mentioned only leos with known and proven genetics should be bread. No way of knowing what you'll end up with otherwise. I've been enjoying reading up on it though, lots of cool morphs and tons of info out there on strong genetics and ethical breeding. Good luck. Russ
Check out this article for an intro to leopard gecko genetics. There are other articles that explain the basics really well and hopefully someone will post the links here