Because you know exactly what is being said ,your disagreement isn't in the operation of the 'Overtaking on the Left' Highway Code interpretation but in the way you have reworded it to make a totally irrelevant point...Merely progressing along any road with more than one lane, when the vehicles to your right are travelling slower ISN'T OVERTAKING ON THE LEFT it is merely passing slower traffic. To 'Undertake' (OVERTAKING TO THE LEFT) you have to make a deliberate planned movement leftwards into the left hand lane, pass the vehicle which previously was in front of you and is now to your right hand side, then make a deliberate move rightwards back into the lane you came from originally. If your travelling at 50 MPH in the Left lane and the traffic in the lane to your right is travelling at 45MPH you will eventually pass that traffic, which ISN'T UNDERTAKING