AFAIK it was to keep a semblance of commonality when we first started getting the automated calls - I.e some fleets had PM or Flt Eng calling “100ft, 50, 30...” and other fleets were in transition from the PM calls to the automated ones.
So we end up with all fleets not having a 40’ call, which is a bit undesirable, but it would cost a fortune to change which no one will pay for. And here we are, many years later in that situation.