This is a Standard Set up in the Mid 1900’s Based upon Different “Schedules”.  The Term Schedule Refers to the Wall Thickness of the Pipe, the Higher the Schedule Number the Thicker the Wall of the Pipe.  

All Pipe Schedules of a Given Size Have the Same OD Dimension.  As the Schedule Number Rises the ID of the Pipe Gets Smaller, Yielding a Thicker Wall.  

What is Interesting About Pipe Measurements is That No Dimension of Any Schedule Measures the Stated Pipe Size.  

If You Take a 1” Piece of Pipe of Any Schedule, You Cannot find any Dimension Measuring 1”.