Apparently they were the ones moving the famous transaction of 200k bitcoins. So, yes, for them this breach could look like: "Meh, we've lost some pennies".

http://www.coindesk.com/194993-btc-transaction-147m-mystery-...

Those are all (vast majority) client funds. 5 million is coming out of their pocket.

Who knows if it comes from their own pocket, or from the customer funds? It is just their internal accounting in the end. The same way the banks work.

I think it is phrased that way to set an upper bound without being overly precise.

It's relative. The loss is a single-digit percentage of total bitcoin deposits held by Bitstamp. They're eating it.