Valve has taken the stance that all player statistics should be private by default. This means that Dotabuff only has access to your statistics when you manually enable it.

In order to enable statistics, please take the following steps:

Open the Dota 2 game client
Click the Settings icon
Navigate to "Game", then to "General"
Set the "Share Match History" setting to "On"

For details, and evidence that this post has been merely copypasta'd from the site, vist https://dotabuff.com/pages/enabling-statistics