A question about Rishabham [ri] and Nishadam [ni].

[Vardhaman]

I have been studying about shrutis for some time now and I noticed a few differences in both systems.
I have noticed that Hindustanis use Rishabham at 10/9 while carnatic musicians use 9/8 one.
Same with Nishadam, ie 15/8 vs 243/128.

Now in Natya Shastra it is stated that Ri is of 4 shruti which would mean 9/8 is the right one.
Ni is one shruti lower than the Sa so 243/128 should be right.
But hindustani musicians use it differently and even in carnatic music notes vary according to the raagam.
Could anyone knowledgeable provide some insight?

Thank you.