I think that a child often just sees a motherly figure and labels her mum. Recently I have been trying to be more nurturing with my skids and the youngest has severall times slipped and called me mum. I think it reflects our relationship rather than some huge compliment to me. I can remember forgetting sometimes and calling my teacher mum at that age!
Of course there will always be step-parents and partners who are over the moon at this development and attribute it to the child's level of adoration toward that step-parent and encourage it. Its not adoration, its just whoever's wiping peanut butter off your jumper.
