How can I return an ArrayList in a PowerShell function without the indices becoming a part of the return value? [duplicate]

I noticed the following behaviour in PowerShell 5.1 when returning Arrays and ArrayLists from functions:

Function A {
    [Array]$data = @()
    $data += 4
    $data += 5
    $data += 6
    return $data
}

$data = A

After calling the function A $data now contains 3 Elements: {4, 5, 6}.

Function B {
    [System.Collections.ArrayList]$data = @()
    $data.Add(4)
    $data.Add(5)
    $data.Add(6)
    return $data
}

$data = B

After calling the function B $data now contains 6 Elements: {0, 1, 2, 4, 5, 6}.

The first 3 Elements are the indices of the ArrayList $data in the function B. Why exactly does PowerShell behave this way and how can I return ArrayLists "correctly" from functions, meaning that $data would only contain the elements {4, 5, 6}?

I think this question is might be related to the first answer of this post Returning ArrayList from Function/Script, which mentions that outputting collections (not just arrays) causes PowerShell to enumerate them by default.