Experience shows us that approximately 80% of people use a left handed thumb compass (versus right handed). We offer these sets as 80% left and 20% right for that reason. We try to send compass sets with different coloured straps for left versus right to make it easy for teachers, coaches, and students to select the hand they are looking for.
We also offer the compass sets as all left, to simplify the distributing of compasses amongst a large group of students.
And there is the option to let us know a different distribution if there is a specific distribution you are wanting.