I have what should be a fairly simple script where it spits out computers that don't have a "ManagedBy" user listed. The code is pretty straight forward

Get-ADComputer -Filter * | where ManagedBy -eq $Null > C:\Scripts\ManagedBy.txt

The Results look like I'd expect them to

DistinguishedName : CN=Computer1234,OU=Desktop,OU=Win7,OU=Workstations,OU=Computers,OU=cool,DC=mydomain,DC=com
DNSHostName       : Computer1234.mydomain.com
Enabled           : True
ManagedBy         : 
Name              : Computer1234
ObjectClass       : computer
ObjectGUID        : 5b917740-fdf8-46e7-9a4b-0334534534e5653
SamAccountName    : Computer1234$
SID               : S-1-5-21-165822833-1632583300-13730345345345-226510
UserPrincipalName : 

But when I pull up the computer in AD it has a user listed for ManagedBy. What am I doing wrong?