The blog post Quick note on line spacing discusses line spacing and it says
To change the line spacing for the entire document, you can use the
linespreadcommand in your preamble:
The factor is somewhat confusing. For double-spacing you have to use 1.6 and for one-and-a-half spacing 1.3. Not very intuitive, but I'm sure there is a reason for it.
This unintuitive factor triggered my curiosity. What is the reason for it being as it is?
This question is just motivated by my curiosity and I have no practical reason for asking it. Feel free to close it if it's inappropriate or see at as post in the same vein as \nothing, \varnothing and \emptyset in that it asks about history or some design decision.