in relation to bandwidth 600k stands for 600,000 bits per second.
there are 8 bits in a byte, therefore giving you 75 kilobits per second. as there are 1024 bytes in a kilobyte (notice kilobit/kilobyte) the maximum speed of a 600k line is 73.2kb per second.
anything above that is purely burst transfer.
so don't worry para.. you're getting exactly what you should