If you kill enthralled slaves' masters, the slaves are not freed if they (slaves) didn't see you do it and therefore didn't acknowledge their freedom. They also don't count as freed for the achievement.
Also if your spell speed is more than 100% and you kill the slaver with a spell and then a spell on autocast (when no enemies are visible) triggers and kills just saved slaves of that slaver, those slaves don't count as freed unless they had time to acknowledge their freedom.
[1.5.2] Freed slaves sometimes don't acknowledge freedom
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