It's the difference between saying "It is actually my mother" and "She is actually my mother" in English. You could say both depending on the context.
"Is your best friend Tom?" "No, it is actually my mother."
"Isn't that lady our teacher?" "No, she is actually my mother."
take a look at https://forum.duolingo.com/comment/9708920
"In Swedish, when we want to introduce a new concept, we use the construction det är. det in this construction does not refer to the thing introduced. How could it, since that is a new topic that is being introduced to the listener? It would be very illogical to start out by mentioning the gender or number of a thing that has not even been mentioned to the speaker (or so native speakers of Swedish feel)."