HD Sentinel is reporting the drive's SMART data. A bad sector that has been "reallocated" will have been replaced with a good spare. That's why the surface scan was clean. Nevertheless, once bad sectors begin to develop, then more are sure to follow. With today's drives, the problem is more likely to reflect a weak head.
Don't trust this drive with your important data.
BTW, if it's an Advanced Format drive, then the actual number of bad physical sectors will sometimes be 1/8th of what SMART is reporting, namely 64 (= 512 / 8). That's still a cause for concern, although HD Sentinel may be underestimating the drive's "health".