It is because Militia regiments were significantly different from the “Regulars”.

During the period of Jane Austin’s life The British Empire was almost constantly at war. The Militia arose from a need to raise a home defense force to maintain domestic order and deter invasion, while the regular army was off fighting the French, and (Americans!) not to mention campaigns in India, Germany, Spain the West Indies and just about everywhere else if it comes to that.

The Regulars were professional soldiers and did the fighting that was going on throughout this period overseas. The regular army was serious and had a fully professional officer corps.

The Militia on the other hand were volunteers, who held temporary commissions. Joining the Militia enabled a young man to indulge in a bit of horse riding, strutting around in a uniform and generally having a good time, without the danger of ever having to actually fight! (Except to put down the odd riot by the revolting peasants)

The fact that the commissions were temporary meant that when an office was not on duty or left the corps the right to address by the rank did not apply.