i am of course only assuming... but if SST can pass this helmet for the ANH version, it most certainly could carry on using it for both ESB AND ROTJ... perhaps it was simply a business decision.
If that were the case why isn't there a precedence for it in SS history? Why did SS go to painstaking lengths to get the design on each of the 3 Anakin sabers exactly right when they could have just used one basic Anakin saber hilt design and reproduce that for the ANH and ESB Anakin sabers?