Read this book. Everybody should.
Most importantly the point is moot: Casual gamers are going to play less and casual games are going to be played less than core. What matters is that the casual market keeps the bloated carcass of core gaming afloat.
Next, the precise value given. When ever you see some odd or complex statistical statement (multiple variable: Wii ownership, playtime, last played, competitive market, new users) there are often too many factors influencing users answers and interviewers interpretation. I am pretty sure that the interviewed has little to no knowledge or consideration of other factors: total household use, channel time usage, total household time spent thinking about using the product (which any gamer knows how much this influences future purchases)
Last here, but not for the sake of statistical argument, people who do surveys do not reflect the true population. Internet surveys get a closed audience who answers to prove a point and phone surveys... well, who really answers their home phone anymore? I'll tell you who, people who are so lonely that they can't wait for the next pollster or telemarketer and people who are so completely out of touch with the reality of wireless communication that they refuse to let the wired phone go... like my internet company who forces me to have a phone (which I don't even have plugged in).