Function not returning expected object

There's a implicit pipeline there, and pipelines tend to "unroll" arrays, collections and arraylists one level.

Try this:

return ,$CORules

Or you can force the variable $CORulesResult to an Array with [Array] in front

[Array]$CORulesResults = Get-ContentOrganizerRules $web.URL;
                        if($CORulesResults.Count -gt 0){
                            $Results.AddRange($CORulesResults);
                        }