Wonky Doors in Roupell Street

This may have been asked before but does anyone out there know why the front doors in Roupell Street are wonky? The top of the door frames are all at a slight angle to the horizontal - in some cases leaving a gap above the door. The houses all seem perfectly straight and the street isn't on a hill. Was there a shortage of skilled carpenters when these houseswere built or had they had a touch too much of the local ale? Hope someone can answer this mystery.