Why are the tongue and groove off set from the centre line on floorboards?

So the boards are less prone to splitting when nailed, the thickest recess being the top or face of the board,and nailed knocked in at an angle between the tongue and the corner of recess helping to hide the nail head

Because you are going by the face of the board. the tongue or groove will or are there, to receive the next board. When the two are joined the boards continue to run parallel, simply by the fact that the tongue or groove act as a guide for the next board.

The thicker part of the board allways goes to the top this is to allow for sanding if or when they are to be coated and left on show or if the board gets damaged they can be repairedalso the top part of the board will come together before the bottom so that you allways have a tight fit

So that you know which way up they should be fitted. The larger cross sectional area should be at the bottom, giving more strength to the floor.