Indeed they seem to use the same image identification ID:
define Device/asus-rt-n66u
DEVICE_TITLE := Asus RT-N66U
$(Device/asus)
PRODUCTID := RT-N66U
endef
TARGET_DEVICES += asus-rt-n66u
define Device/asus-rt-n66w
DEVICE_TITLE := Asus RT-N66W
$(Device/asus)
PRODUCTID := RT-N66U
endef
TARGET_DEVICES += asus-rt-n66w
(hint: PRODUCTID := RT-N66U).
I guess noone noticed this when we were adding these identical images.