Taking a stab at this.
I'm assuming it's because a MAC Address wouldn't directly be recorded to the node as much as it would to the interface.
Stick this in a Orion Report Writer Advanced SQL report and see if you can get closer to matching up what you want.
SELECT N.NodeCaption AS "System Name"
, N.AgentIP AS "IP Address"
,I.Caption AS "Interface Caption"
, N.SerialNumber AS "Serial Number"
, I.PhysicalAddress AS "MAC Address"
FROM dbo.NCM_Nodes as N
join dbo.interfaces I on N.CoreNodeID=I.NodeID
WHERE N.SerialNumber IS NOT NULL
Order by N.NodeCaption