Annulled and divorced - why HDB's different treatment?

Recently, I went through an annulment and have been trying to retain the flat bought with my former wife.

HDB policy states that if a marriage is annulled, the flat registered under the couple's name must be surrendered to the HDB unless my parent's name was in it when I applied for it.

If a childless couple divorces, the divorcee may add his new wife's name or his parent's name into the flat to retain it.

I cannot see the reason for the difference in treatment of the two.

Why shouldn't someone who has had his marriage annulled not be able to add in his future wife's name to retain his flat?

Suneshkumar Mahalingam