我想問.......=.="

The most possible reason is about your habbit:

Here i see a little trend:
all actions that your right arm strong at is involving extending your forearm, while most actions (except shooting a basketball) that your left arm strong at is about bending your forearm

Note that the two muscles of our upper arm contribute to different actions: bicep for bending the forearm and tricep for extending the forearm (please see the websites I quoted for further information)

So once you have developed a habit of using your right arm for actions of extending your forearm and using your left arm for actions of bending your forearm, they began to grow at different rates for different arms.

For the case of writing with right hand, it is simply because your parents or teacher teach you to write with right hand first, and then develop such habit

*** For the special case of shooting basketball, i guess the reason is quite interesting:
after you spend much efford on bouncing and controlling the ball with your right hand, your right hand feel tired.

What i write above is just my suggestion, the actually reason behind can be much more complicated! but I really appreciate your observatory and questioning sense ^^

2006-12-31 22:19:57 補充：
sorry that i dont know how to type chinese
but i will learn it soon

我有個朋友寫字用右手, 但食飯就要用左手, 兩隻手都好靈活, 我o地仲笑佢可以一路食飯一路做功課, 咁就慳番好多時間, 效率亦高好多. 咁即係你o既能力同反應都會比其他人好囉! ^-^