Odysseus is known for his cunning, but he’s not the only crafty character in Greece. Calypso is a sorceress and has a few tricks up her sleeve herself. Odysseus has been a captive of Calypso for 7 years and it is only after she is commanded by the gods to release him that she considers the idea. So, while Calypso has all kinds of supernatural powers at her disposal, it is good old fashioned reverse psychology that she attempts to use on Odysseus to compel him to stay with her. Since she has no choice but to release him anyway, she tells Odysseus that he is free to go in the hopes that now that he is free he will choose to stay. By making it sound like her idea she sounds like the good guy in the situation and hopes to win Odysseus’ favor. Also keep in mind that she attempts to win him by offering immortality as well.
Well, anyone who knows Odysseus knows that he has a clever tongue. When he refuses Calypso’s invitation to an immortal life (a tempting offer for anyone) he seems to pacify Calypso by admitting that the wife he returns to (Penelope) could never compare to the splendor of the Goddess herself. It may be that this flattery goes a long way to soothe Calypso.
It’s also important to note that the speech Calypso makes about the double standards of the gods earlier in book 5 establishes that she has bigger fish to fry than Odysseus. Her problem is really with Zeus and the male dominated order of the world she lives in. She may be distracted from the snub by Odysseus by her greater anger for Zeus, or it may be that she’s to a degree accepting the order of the world she lives in and not raging against it by unleashing her frustration on Odysseus. Not to mention, she’s been commanded by Zeus to let Odysseus go, to disobey would spell great trouble for her.