We don't know the real numbers or odds on rolling heroes with gems, only IGG knows.
When there is an event like "Hire Heroes and Win", rolling legendary heroes must have higher chance or odds.
From what I've read here http://castle-clash.wikia.com/wiki/Heroes rolling legendary heroes have a 5% chance (but who knows), now, when there is "Hire Heroes and Win" the chances to roll a legendary hero must be higher like 10% maybe or more. It's very painful to see greens a lot, also has a higher chance for the "Rare" Legendary heroes to be rolled.
( i know i sounds stupid)
Not to be PERFECT.