Part of the reason may have been the change in sales forecasts for Vista versus XP, in which last month they decided that XP will sell signicantly more than expected:
Last month, the company's chief financial officer said that he tweaked the fiscal year 2008 forecast to account for XP's longevity. Rather than count on an 85/15 split in sales between Vista and XP, said Chris Liddell, Microsoft now expects a 78/22 split, an increase of nearly 50% in anticipated XP sales.
There's one bad side effect to XP's life extension. It seems that XP has nearly run out of available license keys, and MS will run out of keys to issue sometime next year. To combat that, MS is creating a new build of XP specifically to fix this. The new build will not add or remove anything from the OS, merely change the type of keys it accepts:
The new build, dubbed SP2c, includes no fixes or feature changes, but was created simply to address the shrinking pool of product keys. XP Pro SP2c, which has been released to manufacturing, will be made available to OEMs and system builders next month, said Microsoft.
Perhaps Vista is not doing as well as Microsoft would like to claim?