Must have been taken just after some large trade. Here are my samples from the time around my post (timestamps in CET):
20:12 mtgox premium over bitstamp is currently 9.43015538326 %.
20:53 mtgox premium over bitstamp is currently 9.80714742711 %.
22:18 mtgox premium over bitstamp is currently 9.15855226897 %.
Currently:
07:18 mtgox premium over bitstamp is currently 9.95425434584 %.
Far below the 12% to 14% from a couple of days ago.
That is a lie. Those numbers are completely fabricated, or chosen as the lowest values in a minute by minute range. Mt.Gox/Bitstamp variance hasn't been lower than 10% since 8/9.
These values were taken from Bitcoincharts.com. Would you like to provide your source? I'd love to see it.
I thought my source was well known, and I already provided it. gribble is a bot in #bitcoin-otc and other Bitcoin related channels on freenode. It fetches the tickers from both exchanges and calculates the difference. It works with several exchanges. E.g:
09:35 ;;premium bitstamp btce
09:35 bitstamp premium over btce is currently 2.01656900384 %.
The samples were taken at random times. The samples in your graph seems to be very far apart, and is certainly not accurate enough to prove anyone a liar.
The most accurate way to plot the variance would be to take daily weighted price. Choosing the smoothed data is much better than plotting variations on the minute by minute or hour by hour.