by Randy Thornton.
Since you are on 2.5.1, you might check on the way in which Password salting has changed since 2.4: http://docs.moodle.org/25/en/Password_salting and note the PHP version issue mentioned at the end.
The same exact password on two different sites would be salted differently so the hash would be different.
If you are going Moodle-to-Moodle, have you looked into MNet as an alternate method?