Answer: Because, the parts in 1/8 is doubled than 1/4, so, we need to double the copies.
Explanation:
Since, by the below diagram,




Thus, the parts in 1/8 is doubled than 1/4, so, we need to double the copies.
That's why we take 2 copies of 1/8 to show the same amount as 1 copy of 1/4.