The EU says the Single Market adds 2% to GDP, so how could leaving it cost 6%?

'To summarise: the European Commission says being in the Single Market adds 2% to GDP. If we were outside, we could do additional trade deals with the rest of the world, beyond the three significant countries with which the EU currently has deals: Mexico, South Africa and Korea. 

The European Commission's own analysis suggests that just seven of those deals would be worth 2% of GDP, and the three the UK would be most likely to get that the EU would not would be worth 1% of GDP. And all of that is before one even touches upon other gains from being outside the EU, such as allowing the EU to work better and grow faster, improving UK regulation and repatriating the UK's net contribution.

Since the Single Market adds 2% to GDP and leaving it would allow us to pursue trade deals worth more than 1% of GDP and perhaps much more, how can the Treasury seriously expect anyone to believe leaving the Single Market will cost 6% of GDP?'

More here http://www.andrewlilico.com/2016/06/06/the-eu-says-the-single-market-adds-2-to-gdp-so-how-could-leaving-it-cost-6/ but not on the institutionally pro EU BBC.