I think 10^3 is used for KB but it is for bandwidth & 2^30 is used for GB it is for data.
see data is always in binary form so we have only two no. that is 0,1 that’s why we have used 2^n for data bits & for bandwidth it can’t be in discrete form it can be any no. that’s why we have used to 10^n for bandwidth.
that is decimal system is used in bandwidth & binary system used in for data.
simply in Decimal SI (base 10), 1 Kilobyte is equivalent to 1000 bytes. On the other hand in Binary (base 2) system, 1 Kilobyte is equal to 1024 bytes.