I won't do it and you can't make me

I know that my little girl will do things in her own time, she will do all the other things that other babies do when she is good and ready to do them.  I know how far she has come, to barely being able to hold her head up at 5 months old to holding it up just fine now.  I guess I see other babies online that are younger than her that are sitting, crawling, standing, walking even and I start to feel sad for her.  I am not sure why, I mean I know she is just fine and she is not sad about it, I know that when she is ready she will do it.
She has become very stubborn,she will talk and babble while the physical therapist is here, she is really vocal.  But the minute the speech therapist walks over the threshhold, mums the word.  I kid you not, not a sound, not a rasberry, NOTHING.  The therapist tries anything and everything to get her to make some noise, and she's like "Nope, not today lady."  How does she know?  It's like she knows exactly what she wants her to do and she will not do it.  "I will talk when I want to talk lady and I don't want to talk right now, so there"  The minute the speech therapist leaves it's like the flood gates open, and she has so much to say, she really can't contain herself.  It really is too funny, boy am I in for it when she is a toddler, a teenager.