Because more sellers default to exact match or phrase match campaigns — seen as easier to manage since they don't require the same negative-keyword upkeep as auto campaigns — the same search term can sometimes be won more cheaply through an auto campaign than by bidding on it directly via exact or phrase match. Fewer competitors bidding through the auto channel for a given term means lower effective CPC for the same placement.
This is a separate reason to run auto campaigns beyond keyword discovery: even for terms a seller has already identified and validated, testing them through auto (once surfaced in the search term report) can be cheaper than moving them straight to exact match.
The cheaper CPC happens because most sellers concentrate their competitive bidding inside exact and phrase match campaigns, leaving comparatively little competition inside the auto marketplace for the identical search term — so the same term can cost less to win via auto than via exact/phrase.
Из тем: PPC Campaign Structure & Bidding