Growth estimates and the IMF

This might be the most tedious title of any post here, but the recent IMF upgrade in growth estimates for the UK set off a thought process.

The forecast for UK growth this year received a significant upgrade to 1.4%, up from July's estimate of 0.9%.

Here's the thing: UK GDP grew 0.4% in Q1 2013 and 0.7% in Q2 2013 (Q2 figures may be upgraded further). In other words, when the IMF predicted total growth in 2013 to be 0.9%, the UK had already exceeded that figure, in the first half of the year. So, they upgraded their estimates. But they only upgraded them so far as to predict 0.4% growth for the two remaining quarters combined.

The evidence we have for Q3 is that growth was substantially quicker than in Q2: services, manufacturing and construction PMI figures have basically gone gangbusters for July, August and September. The BCC is predicting Q3 growth of up to 1%. By my reckoning that would make growth of roughly 2% in the first three quarters. Given that all the figures I've seen are obviously available to the IMF, what do they know that we don't?