The statistical "submission" is only done when the virus warning is displayed.
That is - not in silent mode, not even when you check the "Don't show again" checkbox during an on-demand scan and further viruses are discovered (during that scan).
However, it's on purpose this way (first, we don't want to submit anything without the user's knowledge/confirmation, second, we don't really want to receive the statistics for every detected sample e.g. in an av-comparatives.org test
).
Considering the number of avast! users, it can hardly affect the statistics in any measurable way.