Grossman was headhunted from the UK where she had headed Britain's Pension Service. The Herald on Sunday revealed in June that she had suddenly left the role.
Chief executive Brendan Boyle said the new recruit had "resigned for family reasons", and accepted two days' notice.
At the time questions were raised about the hasty departure, especially because of the entitlement of up to $50,000 in relocation costs to bring Grossman and her family to New Zealand.
Bennett told Parliament there was no conspiracy behind Grossman's departure.
"She handed in her notice because, as far as I was informed, her husband has had job opportunities in the UK and she wishes to return back there," Bennett said.
Rumours of a personality clash between Bennett and Grossman were quashed by Bennett who also told the House she was "saddened to hear of Janet Grossman's resignation, but I wish her well in her future endeavours".
The project to set up the Work and Income's public self-service kiosks began before Grossman started, but the development and rollout was finalised in October, on her watch. Major security flaws allowed public access to corporate and private information for 1432 individuals. The ministry is conducting four employment investigations following a report into the privacy breaches.
But Labour welfare spokeswoman Jacinda Ardern said if Grossman did receive the $97,000, it would be an extremely costly appointment on top of headhunting and relocating her.
Ministry of Social Development chief executive Brendan Boyle would not provide a breakdown of the $97,000 payment. He said five senior managers left this year and it would be "extremely presumptuous to assume" it went to one person.