Working in SCM, I see many factors that affect the volume of headlights. But, projected VORQ (vendor order replacement quantity) can make the biggest impact. It's highly possible that, initially, the headlights were the same cost, as the tooling for both is identical.

Statistically, more RH headlights die an untimely death each year than LH lights (depth judgment of the average American driver is poor... RH light is farthest away). Since more of them are replaced, the set price point can be lower because, for each die setup you sell X number of units. For the LH, it's X-y therefore the price must be artificially inflated to recoup the difference in sales volume. Assuming a $5,000 setup cost per retooling for each run, the 25 RH headlights will be sold for less $$$ than the 20 LH headlights that were produced.

If you simply make the same number of each, you risk losing valuable PDC inventory space by having to stock large volumes of a slow-mover. Slow-moving parts (Stock Code 4 @ Nissan) are the money-suckers of any PDC operation, as the usable storage space is cluttered and the fast-movers must sit on a trailer for additional hours, as opposed to being off-loaded and imposted into the PDC's inventory, and ultimately fast-flowed out of the building to the dealers (sales).

Hope that made sense