Maybe this helps a bit:
https://en.bitcoin.it/wiki/Transaction_feesBut: According to that, it shouldn't have charged you a transaction fee, since the transaction is above 0.01 bc:
0.01 BTC fee if sending any transaction less than 0.01 BTC. This is to help prevent DoS attacks against the network.
On the wiki there's another part when you will be charged a fee, based on the kb size of the transaction - but i still don't get that part, so can't explain.