kbps=kilobits;
KB/s=kilobytes

8 bits in a byte, soooo . . . .

52 KB/s x 8 = 416kbps

Hey! You are doing better than you thought!

Actually, the KB/s can sometimes be higher, for a moment, than the actual transfer rate (because of averaging).

The tested speed from ISP check sites can vary too, because of loads at the time.

At any rate (pun intended ), your connection sems to be doing OK.