it isn't supposed to be anything, but the fast the modem the faster your connection. 56k is the fastest speed they have right now. (Your connection speed may be slower than the actually speed of the modem)
Well you might as well get a 56K modem since
1. dialup technology as such has reached maturity and wont be developed anymore
2. its bloody impossible to get anything else really (its not but you might as well since 56K is cheap)
56.6k is a theoretical speed, its the actual rated speed of two-twisted pair wires, this is assuming that you have no overhead (which all lines do), and after you add in the noise on the line (which all lines have also) the rate is usually somewhere around 48k at the most