Suspicions are cheap and worth less
Whether or not 0.07% or 7% is significant is a difficult question that depends on the amount of data and how accurately it is measured. Your suspicions about the "margin of error" are therefore baseless. They might be reporting on samples in the millions where tiny fluctuations can be measured.
Having said that, it is the Register, and I'm certainly not going to invest or gamble any money based on their reporting. To me, the real focus of the Windows XP topic should be that Microsoft's threats and blackmail aren't working perfectly, and as too often happens, the Register is off chasing some codswallop.