Nick says Feed Demon defaults to checking once every three hours, so depending on how you look at stats it may appear 1/3 of readers that poll every hour, skewing its market share numbers. Something similar may effect WordPress’ numbers with installations and blogs. If the average MT user has two or three blogs per installation (like their survey indicated) then the actual number of installations would be half or a third of what the blog survey numbers indicate, putting it much closer to WP’s numbers. Of course I know of no good way to track this, so it’s just a random musing.