And these figures mean what? A little clarity of context would help. Does it mean the average usage over the whole sample or the average usage in homes where the listed console is present? I'll guess the former.

No it's by total play time across the saple. So the occurence of the console is not measured.

One guy playing Xbox 24 hours a day would result in the same statistical result as 48 guys playing PS3 30 minutes per day.

And they don't "measure" this stuff empirically. It's produced from poll responses, where they ask people what console they have, and then ask them how long they spend playing it each day. People's recollections are likely to be inaccurate for a number of reasons, not least because immersive gameplay distorts the perception of time. But the inaccuracy should be consistent across all platforms, which is why this data is considered acceptable.

Increased on-line gaming and cable/satellite viewing through Internet connected set top boxes should render these kind of guestimated statistics obsolete in the future.