Machan this is a difference between KB (KiloBytes) and Kb (Kilobits). As you know there are 8 bits for a byte, so the maximum download rate you'll get under 512Kbps connection is 512Kbps / 8 = 64 KBps.
You get the of 511Kbps in your line as shown in the Prolink router page.
According to that...