I think that girls act more tomboyish or less so as the result of a few things:

1.  Which parent and other relatives they most enjoy spending time with and which they resent.  (Enjoying to spend time with one's father and/or brother while despising one's Mother and/or sister, and the other way around).

2.  Which gender they are most accepted and which they are most rejected by.  (Friendship vs. Bullying)

3.  What activities they have access to and enjoy most.  (Ex:  Having little sports equipment to play with but lots of dolls vs. having a lot of sports equipment to play with but few dolls.)

4.  Some biological factors which form a sort of basis for everything else. (Ex: The brain develops more masculine or less masculine characteristics).

5.  Personal success vs. personal failure (Being more successful at an activity vs. failing at it would likely result in one enjoying or despising the activity and thus, be more or less inclined to do them).
And finally,

6.  The behaviors as a result of the above and how they affect what happens with them.  (Ex:  Acting more or less typical of gender role will affect how others react to you and in turn, how you react to them, further altering one's behavior and creating a sort of cycle.)

I also believe these factors affect how one reacts to the genders (accepting and rejecting stereotypes, as well what gender one prefers to spend time with) and even one's behavior in general but, that's another matter entirely.

Yes, I know I have no citations, but this is what I derived from personal experience and a little common sense.  Even if this is true, I have no idea where to get the citations for this.