Call a Variable Before Assigning It

I have a script that sends an email message to users and use splatting to set the send-mailmessage parameters. My code in brief form looks like:

$NotifyMessage = @{
                SMTPServer = $SMTPServer
                To = $NotifyTo        
                From = $From
                Subject = $notifySubject
                Body = $NotifyBody
                BodyAsHTML = $true
                }

Get-aduser -filter * -properties * | foreach {
$NotifyTo = $_.mail
Send-MailMessage @Notifymessage
}

As you can see I have set the -To parameter of the send-mailmessage cmdlet using the $NotifyTo variable prior to actually assigning the variable. When initially testing the script I didn't expect this to work but found that it did. However, when I tried to run this script via Scheduled Tasks no message was sent. I then moved the splat table assignment into the foreach loop below the $NotifyTo assignment and it worked. Can anyone expain why this behavior occurs? Is this a scoping issue or something else I am not seeing? Thanks in advance!