I think there is a bug with "last trades" using Bitstamp. I see a lot more trades at a given level than are showing on any of the other live web tickers.
I think it just keeps on repeating the trade every few seconds until another real trade happens at another level. For example

It shows 0.99 @ 347.81, then that order keeps repeating several times until another order comes along at say 0.50 @ 348

Watch the it against http://bitcoinwisdom.com/markets/bitstamp/btcusd and you will see what I mean.

I did have a look at the MtGox "last trades" and it does not do this so I think there is something up.