My money's on a combination of the scar tissue and the actual amazing nature of the human body.
Human children can completely regenerate digits to an age of about 3-5 years (As per my pediatrician uncle)...since you were about 7 when the damage occurred, I'd say that the regeneration of a tendon or ligament is reasonable, since the rest of the finger was actually there.
And if the bones healed up funky with lots of scar tissue in the surrounding soft tissue, it could have impaired movement. Introduce some blunt force trauma, and it breaks up some of the stuff, and VOILA!! Full mobility! That's my guess. Still freakin' incredible though!
Or you could be a zombie.