Security disabler doesn't operate in context of Unlock-Item comandlet?

Question

Writing a script to allow editors to unlock an item locked by another user I used

New-UsingBlock(New-Object -TypeName "Sitecore.SecurityModel.SecurityDisabler"){
$item | Unlock-Item
}

This didn't raise errors, but didn't unlock the item

Directly calling the Unlock() method did work though

New-UsingBlock(New-Object -TypeName "Sitecore.SecurityModel.SecurityDisabler"){
    $item.Locking.Unlock()
    }

Is there a reason for this?

Note: this is only the briefest of outline, other possible differences include retrieving the item to unlock inside or outside the security disabler block, and getting multiple items (using a foreach-object for unlocking by second method, piping the results of the query using the first).