42 inch mower deck Spindle Nuts- WHY SO MANY ???

[Lane Ranger 10,576]

I was ordering some new 42 inch mower deck spindle nuts before changing and sharpening blades.  These usually get pretty banged up after lots of use so I try to replace with new ones to be easier to remove the next redo.  
 

 

But I never really noticed before the variety of numbers used on the spindle nuts.  I am sure there is a reason but I have found the following numbers: 3220-7, 905090-4, 915007, 3219-7 , 9220257, 

 

 

I see some are jam nuts but all seem to be 3/4 X 16 threads.  

 

 

One note on a 42 inch Mower Deck Manual says the 915090-4 Nut   ending in 4 is that the 4 means -Free Running Nut 

 

 

 

Edited April 5, 2023 by Lane Ranger