From MTG Wiki
How are they twice as rare if they are found in 1 in 8 packs?
- Because there are fewer mythics in a set than there are rares. E.g. Zendikar had 15 mythics and 53 rares. Each booster had a 7/8 chance of getting one of the 53 rares, and a 1/8 chance of getting one of the 15 mythics. So each rare appeared in 1.65% of Zendikar boosters, and each mythic appeared in 0.83% of Zendikar boosters. So any given mythic is seen half as much as any given rare. --AlexChurchill