NMHDR definition conflict?

in NMHDR is defined as follows:

typedef struct _nmhdr
{
    HWND    hwndFrom;
    UINT    idFrom;
    UINT    code;
} NMHDR;

in , it is defined as :

typedef struct tagNMHDR
{
    HWND      hwndFrom;
    UINT_PTR  idFrom;
    UINT      code;         // NM_ code
}   NMHDR;

note that IdFrom went from being a UINT in richedit.h to a UINT_PTR in winuser.h

when compiling for 32bit that won't make a difference but, when compiling for 64bit, the winuser.h idFrom is going to be 4 bytes larger than the richedit.h idFrom.

I suspect that I must be missing something because a difference in size of 4 bytes between the two structures would have broken a fair amount of code out there but, I don't see how that conflict is resolved.

I would appreciate an explanation of how these two seemingly incompatible definitions can coexist. Thank you for your help.