Why ' len equ $-aa' doesn't work in source addres in assembly(emu8086)?

data segment
    aa db 22h, 22h, 22h
    len = $-aa    
ends

stack segment
    dw   128  dup(0)
ends

code segment
start:
    mov ax, data
    mov ds, ax
    mov es, ax

    mov si, offset aa

    mov [si+len],al
    mov al,[si+len]   


    mov ax, 4c00h ; exit to operating system.
    int 21h    
ends

It works like this:

MOV [SI]+03H, AL 
MOV AL, 03H (This is wrong)

when len in source address, the source address will be compiled as the value of len(3)

but when I change the len in data segment:

len = 3

It will work correctly:

MOV [SI]+03H, AL
MOV AL, [SI]+03H

why?