From what I know, and from logical dictation, the reasons line infantry do +1 is for NWL style 1v1s. The reason why it is done this way is to give those with more numbers the slight advantage, however it's not too strong due to the fact that the team with the less numbers has the opportunity to even numbers via musket shooting. When it comes to every line infantry groupfight however, it has always even numbers, because in a pure melee scenario, that +1 adds an advantage somewhere in a melee, when in an NWL style match it could have been negated through better shooting from the team of lesser number. Just food for thought.