Yes, but that's because you (read: FM) don't see the real signature, just what the bootloader returns. And the bootloader is written so that it does not read the real "silicon" signature (AFAIK there is no way to do it), it just simply returns a constant:

IAP_ReadID:
	mov	a,dpl		; 0390   e5 82      e.
	cjne	a,#0,IAP_ReadIDX1; 0392   b4 00 03   4..
	mov	a,#0bfh		; 0395   74 bf      t?	; Mfg ID
	ret			; 0397   22         "
;
IAP_ReadIDX1:
	cjne	a,#1,IAP_ReadIDX2; 0398   b4 01 03   4..
	mov	a,#91h		; 039b   74 91      t.	; Device ID 1
	ret			; 039d   22         "
;
IAP_ReadIDX2:
	mov	a,#4		; 039e   74 04      t.	; both ISP and IAP version number
	ret			; 03a0   22         "
;

And NXP apparently burns the same bootloader into both chips.

JW