Volley and back sights on MLE and CLLE: why the difference?

I have an MLE and CLLE in my collection, both private purchase target rifles built by BSA at some unknown date. They both have the dial sight plate still attached, 1600 to 2800 yards. The dial sight plate on the MLE is stamped with the Roman numeral II, the CLLE's plate is stamped LES2. I was researching if the changeover from the Mark VI to the Mark VII round was denoted by other means when I chanced upon a little tidbit of information from Mr. Skennerton's book: the dial plates for CLLEs are graduated from 1700 to 2700 yards, not 1600 to 2800 as with the MLE. Further, the back sights also have different graduations between the two rifles: sight leaf marked to 1800 yards on the MLE, marked to 1900 on the CLLE. Was there a cogent reason for rifles with the same length barrels firing the same ammunition manufactured by the same manufacturer having differently graduated sights? Or is this simply another Lee Enfieldism?

Nice collection of rifles, especially the Mk III Ross! Regarding your question as to the sighting, the answer is fairly simple; MLE's were sighted for Mk 6 ammunition, while CLLE's were sighted for Mk 7 ammunition. BUT.... Some CLLE's, were graduated for both Mk 6 and Mk 7 ammunition. If you read Skennerton's description of the CLLE MK I*, he states after the introduction of the Mk 7 ammo, some rifles were adjusted for the new load. One should infer that some, but not all CLLE's were re-calibrated for Mk 7 ammunition.

The Mk 6 load was an older load (215 grain bullet, approx 2000 fps). The new Mk 7 load fired a 174 gr bullet, reaching approx 2470 FPS. Enough of a difference in ballistics to necessitate a sight change.

Case in point: I have two CLLE MK I*'s; both converted by VSM in 1909. One is calibrated for Mk 6 loads (like the MLE), while the other is re-calibrated for Mk 7 loads. Those rifles that were not upgraded still have rear sight beds graduated to 500 yards. Volley sight plates remain marked with "II," and are still graduated to 2800 yards.
Upgraded rifles have the rear sight bed marked to 600, the volley plate graduated to 2700 yards, and those plates are marked with a "CL." (see attached photos)
Rear sight ladders were also re-graduated. CLLE's sighted for Mk 6 have there rear sight ladders graduated to 1800 yards, whereas the CLLE's sighted for Mk 7 are graduated to 1900 yards.

The LES 2 plate was fitted to the SMLE series of rifles (short rilfes) utilizing Mk 7 loads.
It would appear your rifle has had its original volley sight placed with one from an SMLE. Perhaps it was missing it's original, and someone plugged in a shaved LES 2 plate. When the volley arm was ground off, the surrounding wood usually bears some evidence of this as well. This is not uncommon, as I have seen many rifles with a variety of plates installed on the wrong rifles.
The Long-Lee volley plate with the "II" mark turn up on the various auction sights with some frequency.
Hope this helps!