OK folks it's no magic.
deftasks.xml is not a configuration storage. That is, the configuration is not read from it when being used.
Rather, it's a tool that lets us
enforce certain settings to your current config. I.e. it is read and parsed every time avast starts (on system boot, usually), and all settings from it are transfered to the actual config storage.
The reverting-to-the-original-version thing you're seeing is a consequence of the fact that deftasks.xml is normal part of avast installation and therefore the avast auto-repair feature always replaces it to the original version. However, this is not a problem as the "import" procedure from the modified version is already done at that moment.
I hope this clears it a bit.

Vlk