Blowfish decryptString encryptString not match?

I use Blowfish to decrypt the encrypted password got from database, then I get the right plain password.

But I encrypt the same plain password using the same blowfish, the encrypted password is different with the database.

What's the problem?

Is there any way to change user's password in a plugin?

I use "AuthFactory.encryptPassword(password)" to get the encrypted password, but it different with the one encrypted by "UserManager.createUser()".