I can write finer then any girl I've every known. Done tons of callegraphy and stuff. Yet, half the time when I'm just jotting crap down, it's barely legible and hte other half it's fairly neat. It just depends. Here, think about this...
When was the last time you saw a boy in elementary school using the back of his spiral notebook to write down the names of all the girls he had a crush on. Decorating it with pretty flowers, peotry, and other useless cute junk? When was the last time you saw a little girl drawing a war scene in their notebooks of bombs dropping fro above, and transformers stepping on flowers, and explosions...
Yah, practice starts at an early age. Girls tend to write pretty because they focused on that while young. That is not always the case though and it is becoming less and less and the society is churning more girls out young with the male mindset to learning.