Why use the older 7.62x 39mm for the battle rifle ? AFAIK the current flavour of military cartridges atm is the Russian or US 5.56mm. Seems a pretty illogical move for a rifle purportedly for the export market.
Important to note that it is a modular rifle and can fire 5.56mm, 7.62 x 39mm and the 7.62 x 51mm. This not only allows the rifle to fit more roles, but also fits the logistics of different countries. It is most likely that these rifles are made to order and have a low rate of production ordinarily, if a country wants the modules for whichever calibre, they will place the order for the modules that suit their needs and logistics and then the factories will produce more of those specific components for each different calibre. Note that they only need to manufacture some of the components for that specific calibre, with things like the stock, grip, magazine housing, etc. remaining the same.
Having another calibre option can't hurt -it doesn't cost much to have a few extra calibre modules designed if it isn't being manufactured constantly anyway.
From a business perspective it will be a balancing act between the sales and the extra cost of equipment should an order be placed for a "less common calibre". If the sales exceed the inability to achieve economy of scale and any delays in the normal production from an odd batch, then it is worth having the option. There is probably a minimum order quantity to ensure that profits exceed the added costs of production.
