Things that matter:
Temp when eggs are in transit.
Freshness of eggs.
Distance eggs travel.
Incubated under a broody vs. incubator
Health/diet of hens.
Letting eggs "rest"
Luck
I am looking at 12 out of 12 Ameraucana eggs developing, from an eBay seller, and in 6 days I should have 15 out of...