Why do my tub drain threads come with different threads? What size of threads will fit my tub?

Typically, all US households are using the universal code of 1-1/2” thread connection (nominal size). When measuring, it may not be the same thread measurements:

The outer diameter (1-7/8” Diameter)– They are nominal threads that are measured by the outside diameter on the drain.

The inner diameter (1-1/2” Diameter) – They are inner threads that are measured by the inner diameter on the drain’s threads.

Example: Tub Drain-DTL201

Because as these fastener mechanisms tightens, they actually flex and stretch similar to a rubber band. Once the ridges and the roots are stretched, it will achieve a sturdy connection to uphold and seal their responsibilities.

See below link for further details on thread concept:

https://www.parktool.com/blog/repair-help/basic-thread-concepts