Reasons for limited dorsiflexion

Can dorsiflexion be limited at the front of the ankle (where the talus
and tibia come together)? I imagine most people's dorsiflexion is
limited by lack of flexibility in the Achilles tendon and nearby muscles
of the foot and lower leg (Achilles, plantar fascia, gastroc, soleus,
etc.).

If anyone wants to test their knowledge of ankle anatomy, I found this:

http://www.csuchico.edu/~sbarker/shock/Anklequiz.html

I got 13 out of 14 right the first try, a result which surprised me,
albeit pleasantly.

What I'm trying to find out is if any significant portion of the
population has limited dorsiflexion that cannot be improved through
stretching. I would like to be able to keep my heels on the ground and
extend my knees _well_ past my toes, and right now I cannot, but I want
to be clear that my dorsiflexion falls within normal parameters, and may
even be above average already. I'd just like it to be better if
possible. I can already barefoot squat deeply (ass within a few inches
of the ground, back flat, etc.)

Thanks in advance.

-S-
http://www.kbnj.com