Why does arrayAppend return true and listAppend return the list?

In ColdFusion, the arrayAppend() function takes an array and an item to be appended. It modifies the array that was passed in and returns true if successful.

The listAppend() function, however, takes a list and an item to be appended, and returns a new list with the item appended. It doesn't modify the list that was passed in.

Why do these functions operate in two different ways? I'm always turning to the documentation to remember the return value of each.