Fasm optimization question about expression calculating

Well, Fasm calculates bot the "AND" and "OR" numerical operators like that:

Code:

      calculate_and:
        mov     eax,[edi]
        and     [ebx],eax
        mov     eax,[edi+4]
        and     [ebx+4],eax
        jmp     calculation_loop
      calculate_or:
        mov     eax,[edi]
        or      [ebx],eax
        mov     eax,[edi+4]
        or      [ebx+4],eax
        jmp     calculation_loop

That was rather simple and as expected. But can someone explain why "XOR" (also "NOT") need such a complicated crap? :

Code:

      calculate_xor:
        cmp     [value_size],1
        je      xor_byte
        cmp     [value_size],2
        je      xor_word
        cmp     [value_size],4
        je      xor_dword
        cmp     [value_size],6
        je      xor_pword
      xor_qword:
        mov     eax,[edi]
        xor     [ebx],eax
        mov     eax,[edi+4]
        xor     [ebx+4],eax
        jmp     calculation_loop
      xor_byte:
        mov     eax,[edi+4]
        or      eax,[ebx+4]
        jnz     xor_qword
        mov     eax,[edi]
        or      eax,[ebx]
        and     eax,not 0FFh
        jnz     xor_qword
        mov     al,[edi]
        xor     [ebx],al
        jmp     calculation_loop
      xor_word:
        mov     eax,[edi+4]
        or      eax,[ebx+4]
        jnz     xor_qword
        mov     ax,[edi+2]
        or      ax,[ebx+2]
        jnz     xor_qword
        mov     ax,[edi]
        xor     [ebx],ax
        jmp     calculation_loop
      xor_dword:
        mov     eax,[edi+4]
        or      eax,[ebx+4]
        jnz     xor_qword
        mov     eax,[edi]
        xor     [ebx],eax
        jmp     calculation_loop
      xor_pword:
        mov     ax,[edi+6]
        or      ax,[ebx+6]
        jnz     xor_qword
        mov     eax,[edi]
        xor     [ebx],eax
        mov     ax,[edi+4]
        xor     [ebx+4],ax
        jmp     calculation_loop

Apparently, it is checking the operand size by testing how much of the upper bits are set/zero. But then, why doesn't "AND" and "OR" need such a thing?

Please help Privalov, or Tomasz Grysztar? (I just see you put your real name in, maybe I should do so also ?) )