That means that there are some bad sectors on the drive and they were reallocated.
Some HDD testing tools report normalized values while some report RAW data. Most probably because of that, some tools report that there are no issues with the drive(there should be more than 56 reallocated sectors in order to warn you) while others report the exact reallocated sectors value and show a warning sign as a result.
Anyways, in case the drive is under warranty, you can just contact the manufacturer and eventually RMA it.
In case it is not, you just shouldn't store important data on it. My guess would be that bad sectors will eventually continue to appear.
Here are some other tools you can try:
http://www.tomshardware.co.uk/forum/282651-32-best-diagnostic-testing-utility
D_Know_WD