clarify this for me please

So I have $500 deposited in my live account with oanda and set to 1:50 leverage. I opened a short position on AUD/USD at 0.7657. I utilized 2500 lots (.25 cents/pip). I put my stop at 0.7697, 40 pips above that implies that if I lose this trade, I will lose 10 bucks, which will be 2% of my equity.

However, I would like to know the account indies that I used 38.18 of their margin leaving 466.78. I simply don't know that. I thought that this trade is only going to utilize 0.50 cents of my equity to open (2 pip spread on aud/usd). Can someone clarify this for me?!?!?!!?!

The Oanda margin explanation is loed here:

http://fxtrade.oanda.com/fxtrade/margin_rules.shtml