Typically Exchanges execute orders at price, then time priority. So your order should have been filled before any subsequent orders.
Were you looking at the public market data to determine who came first? or looking at Bitstamp's acknowledgement of your order?

Specifically with Bitstamp, we've seen that there is a lag between the time your order is acknowledged as Submitted to when it appears on their orderbook. What may have happened is that the orders that were executed before yours were actually sent in before as well.

The order of events might have been:

1. Other market participants send orders at price $400
2. You send in your order for the same price
3. Your order is acknowledged by Bitstamp
4. Other orders appear on public order book
5. Your order appears on public order book
6. All orders are executed in Time Priority, yours is filled last