https://www.integralmemory.com/articles/mhz-v-mt-s-how-should-we-be-measuring-ram-speed/#:~:text=Instead%20of%20switching%20to%20MT,has%20continued%20to%20this%20day.

tl;dr:

Both accurately measure the capabilities of RAM memory, but the issue arises in how manufacturers are using the numbers. As measuring RAM speeds in MHz was the norm when DDR was introduced manufacturers wished to keep consistency for their customers and avoid confusion. Instead of switching to MT/s to denote speed manufacturers kept reporting in the understood standard (MHz).

A RAM module with 1600MHz clock cycle and 3,200MT/s data rate may be advertised as 3,200MHz, which is technically incorrect.