Microsoft's Chris Lewis has told MCV that the European install base for Xbox 360 is one million higher than the one for PS3.
According to recent figures, 360 has 9-million-plus and PS3 around 10-million-plus. His explanation for how Microsoft is bigger, goes thus:
"We’re confident we are actually around a million units ahead. We count installed base differently to them.
"We count the number of units in the hands of consumers. We are committed to continuing that momentum – not only in places like the UK, France and Germany. Continued growth in all parts of Europe is integral to winning [against PS3] overall."
More through the link above.