Is there any reason why the origins of plan and orthogonal (0, 90, 180 270 deg) elevation atlas pages do not produce equivalent grillages?

For example in a layout for a plan view atlas I have the following;

  #ATLAS NAVIGATION PANE ORIGIN LABELS

  #Sets plan labels

  #----------------------------------

  #Choose arbitrary datum for Beautiful Bulmer Series

  # A3 Atlas sets based on existing magnetic co-ordinate grid

  origin 5485 6320 1075 metres   #co-ords of lower left corner of 'origin' atlas page

   origin-label 1 A #label & type <across> <down> 

 and in a layout for elevation view atlas I have;

 #ATLAS NAVIGATION PANE ORIGIN LABELS

  #Sets elevation270 deg labels

  #----------------------------------

  origin 5485 6320 1075 metres   #co-ords of lower left corner of 'origin' atlas page

  origin-label 3 A #label & type <across> <down>

Notice that the origin co-ordinates are the same, and the origin-label is not the same.

If the origin-label was the same I would expect the plan and elevation atlas’ produced to have pages that represent exactly the same part of the cave, but it does not.  As written above the maps produced almost represent the same parts of the cave on each page, but not quite.  I can understand this difference if the elevation were not projected either 0, 90, 180 or 270 deg, but if it is, why the discrepancy?

Is this likely to be something I’m doing wrong or is it a characteristic of Therion?

Bruce