The daughter could be developing a kind of "crush" on you as well as the other reasons listed already.
But yes at this age she needs to have a strong male role model as to how "men/dads" should or can be and how she can relate to them as she matures.

you and the mom might need to make a plan and maybe make a schedule of some sort if the daughter is getting too extreme in her need for your time?? If she gets too clingy??

I guess a lot depends on the bio father or any previous father like role models ??= in the picture, abusive, unstable etc.