At S255 the 6W will be very intense and burn hard at 75IPM or 100IPM, so the burn line will be wide. Also, if you do not have your test piece of wood at a 45D angle and run at the same angle, the beam will be wider traveling in a different direction and burn wider. That is if you rotated the long side of the rectangular beam parralel to a 45D angle.
The laser's beam can not be interfered in any way by the air assist orifice because it will distort it. Is the lens material look like brass, or is it black? If it's brass, it's either the J-Tech or Chinese single element glass lens.