"The total number of UK mobile data connections (which here includes M2M connections) increased by 6.5 million (13.3 per cent) to 55 million in the year to December 2013," Ofcom's report said. "Most of this increase (6.2 million connections) was in the number of mobile handsets that were used to make a data connection (up by 16.1 per cent to 44.5 million as a result of increasing smartphone take-up), although the number of M2M mobile connections also showed strong growth, up by 0.5 million (9 per cent) to 5.6 million during the year."
So that's a total 6.5 million increase, which is broken down as a 6.2 million increase in connections via mobile handsets, and 0.5 million M2M mobile connections, which together add up to 6.7 million.
Ah, there's also another factor not mentioned - the same paragraph quoted ends "As mentioned previously, the number of dedicated mobile broadband data connections (such as mobile broadband dongles and data-only SIMs) fell by 3.2% during the year." That appears to be 0.3 million, looking elsewhere in the document - so there's still a 0.1 million discrepancy, but that could be a rounding error or some other comparatively insignificant piece of data that I can't be bothered to find.