JA when using military titles or not using them is probably doing so on the basis of what she observed in her society.

In Meryton, junior Militia officers are addressed as Wickham or Mr Wickham, or Denny rather than by their title of Lieutenant.

Colonel Forster as head of the Regeiment and Captain Carter may've reffered to junior officers by their surnames and junior officers probably called each other by surname.
Many local people when mixing with all the officers probably picked up this familiarity and called junior officers like Wickham by surname as well.