Whether the Serial Number returned by various WMI queries will be correct or not will depend on the manufacturer of the device. See the answer to this stackoverflow question, Motherboard ID - WMI C++ Reliable? which contains a couple of examples showing that your results can vary.
Some of the variables are whether the equipment is OEM or not and whether the equipment was purchased from a manufacturer who sells to the end customer such as Dell or a manufacturer who sells equipment to resellers who then configure the equipment or if the equipment was assembled from purchased parts.
Those results are processing the properties retrieved by several different WMI queries.
L"SELECT * FROM Win32_SystemEnclosure",
L"SELECT * FROM Win32_BaseBoard",
L"SELECT * FROM Win32_BIOS",
L"SELECT * FROM Win32_ComputerSystem",
L"SELECT * FROM Win32_ComputerSystemProduct",
L"SELECT * FROM Win32_MotherboardDevice",
For instance here is from a point of sale terminal that shows missing serial number information. The intent seems to be that the reseller or dealer or other vendor will provide their own serial number along with other information to basically relabel the terminal.
Connected to ROOT\CIMV2 WMI namespace
Query for SELECT * FROM Win32_SystemEnclosure
SMBIOS SerialNumber: None
SMBIOS Name: System Enclosure
SMBIOS SMBIOSAssetTag: None
SMBIOS Manufacturer: To Be Filled By O.E.M.
Query for SELECT * FROM Win32_BaseBoard
SMBIOS SerialNumber: None
SMBIOS Product: EIN70-SAM
SMBIOS Name: Base Board
SMBIOS Manufacturer: INTEL Corporation
Query for SELECT * FROM Win32_BIOS
SMBIOS SerialNumber: None
SMBIOS Name: BIOS Date: 10/15/13 20:06:15 Ver: 04.06.05
SMBIOS Manufacturer: American Megatrends Inc.
Query for SELECT * FROM Win32_ComputerSystem
SMBIOS Name: GENPOSA-1
SMBIOS Manufacturer: To be filled by O.E.M.
Query for SELECT * FROM Win32_ComputerSystemProduct
SMBIOS Name: EIN70-SAM
SMBIOS IdentifyingNumber: None
SMBIOS UUID: 03000200-0400-0500-0006-000700080009