Why didn't a margin call prevent me from losing more than my initial investment when buying on margin?

Buying on margin refers to buying of securities with cash borrowed from a broker with securities acting as collateral, covering a portion of the risk of the position. This use of margin magnifies the potential profit or potential loss and both exceed the amount of the collateral. It does not mean that the position is risk free.

At 100:1 leverage, the amount of the collateral covers a mere fraction of possible price change, namely a 1% move. In your case, the security moved a mere 4% and that cause a deficit of $30, three times the amount of your collateral.