Enya never writes a bad melody. That's first and foremost her secret. As she goes along, she'll start changing the dynamics, pushing here and there so that not everything is perfectly in unison. It adds a texture you can acquire only from having different voices. The variations lead to interesting quirks. It's an integral part of the Enya sound.
Imagine you had a bank that each morning credited your account with $1,440 - with one condition: whatever part of the $1,440 you failed to use during the day would be erased from your account, and no balance would be carried over. What would you do? You'd draw out every cent every day and use it to your best advantage. Well, you do have such a bank, and its name is time. Every morning, this bank credits you with 1,440 minutes. And it writes off as forever lost whatever portion you have failed to invest to good purpose.