{ $a } は { Write-Output $a } の事
Write-Output に配列を渡した時の挙動を考えればいい

https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.utility/write-output

By default, Write-Output enumerates through collections provided to the cmdlet.
However, Write-Output can also be used to pass collections down the pipeline as a single object with the NoEnumerate parameter.