You are actually seeing a blend of info from the PE32 header (PE field) and the header of the manifest embedded in the assembly (the rest). This is all described in the Windows SDK, you'll need version 8 to get the new 32BITPREF flag. Use C:\Program Files (x86)\Windows Kits\8.0\Include\um\CorHdr.h, lots of comments in this file that describe the declarations.
I'll copy the section that describes the IMAGE_COR20_HEADER.Flags values:
COMIMAGE_FLAGS_ILONLY =0x00000001,
COMIMAGE_FLAGS_32BITREQUIRED =0x00000002,
COMIMAGE_FLAGS_IL_LIBRARY =0x00000004,
COMIMAGE_FLAGS_STRONGNAMESIGNED =0x00000008,
COMIMAGE_FLAGS_NATIVE_ENTRYPOINT =0x00000010,
COMIMAGE_FLAGS_TRACKDEBUGDATA =0x00010000,
COMIMAGE_FLAGS_32BITPREFERRED =0x00020000,
So a displayed value of 0x20003 breaks down into 32BITPREFERRED (0x20000) plus 32BITREQUIRED (0x00002) plus ILONLY (0x00001)