Ma Durga is Sinhavahini. The Tiger had some other significance, yet to be cleared by the makers if they haven't forgotten.
Some images of Durgama is sitting on Tiger too. But in Scripts she is always mentioned as Sinhavahini not Vyagravahini.
The creatives recycled the tiger part from another part of the Shiva Purana. In the story where Parvati does tapasya to shed her dark skin, a tiger comes and sits near her. Parvati thought it must be her devotee but the tiger really wanted to eat her. However as time passed the tiger began to change and eventually due to the grace of Shiva and Shakti, he became a true devotee of the goddess. Instead of thinking of devouring her, he acted as her guard while she completed her tapasya.
After the whole Kaushiki/Kali story ends, Parvati returns to Mahadev along with the tiger. They give him human form and make him one of the ganas. He is named Somanandi and he becomes Nandi's assistant.