Ad Auction

Every time an ad can be shown, Google runs an auction among advertisers whose targeting matches that viewer, that video and that moment. The highest bid does not automatically win. Ad quality and expected performance are factored in, and the winner generally pays the minimum needed to beat the next advertiser down. This is why your CPM moves while you change nothing. Budgets reset, seasonal demand climbs before Christmas and collapses in January, and the mix of viewers you attract changes what the auction is worth. It also explains why the same video earns different amounts in different countries on the same day.