There are at first sight 3 obvious things that make the Williams faster than it was at the beginning of the season:
1. The new chimneys make the BMW engine breathe more, and thus produces some more hp's. Montoya already stated that makes a lot of their advances.
2. Flipups have been changed, as I said in the Nurburgring technical update. In Magny cours those parts looked even more like on last year's Williams than in any other preceding race.
3. Williams have redistibuted the weight of the car since a couple of GP's now. They take ballast to get to 600kg of course, but have now found exactly where to locate it. Many drivers state assure us that any weight chenge greatly influences the car's balance.