Recently, our team has been pondering the way Google determines which campaign to utilize in an auction, especially as multiple campaign types are eligible for the same auction (PMax + Search or Shopping as an example).
Geez, you wonder, is that what all your team chats are about? You never discuss, like, the new PS5 game "Stray" where you navigate a post-apocalyptic world as a cat?
Okay actually we've had both conversations in the last few days.
Regardless, one thing we discussed that I don't see talked about a lot online in regard to Performance Max campaigns, is the "Super Secret Trump Card" Google can play of: "auction eligibility".
What do I mean by auction eligibility?
Well there are many factors that fall into determining if Google can/will show your ad to a specific user making an inquiry. For instance, if your campaign targets the correctly searched for search term, but you are only advertising in Timbuktu and that person is in Siberia, then your campaign will be "ineligible for auction" because they are outside of your geographical network. Hooray!
But, this can bite you. Let's talk bid floors.
Let's say you have a Search Brand campaign set to ECPC and you are bidding $1.20 on a specific brand term [my widget brand]. In Google speak, when someone searches for [my widget brand] then it should go into your Search campaign rather than the eligible PMax campaign... ...but that's where it can get complicated.
If for some reason Google decides to play fast and loose with the auction and has determined that the bid floor for that specific user's auction for some reason should be set above $1.20 then your search campaign is "ineligible for auction" since bid floors are part of this equation.
In other words, your Search campaign doesn't actually qualify for this auction, so what campaign will now pick up that brand term you were exactly targeting? Yup, the PMax campaign that wasn't supposed to.
Please note, the above is an oversimplification.
I don't know all of the things that make a campaign ineligible for auction (there are many), but it's important to understand that there are things like this occurring on the back end that you'll never know about, and could explain why sometimes you may see Performance Max campaigns pick up searches that should be going to a Search campaign (even then, it's difficult to gauge that since you can't see specific search terms in PMax!).
It's definitely in Google's favor to keep everything obfuscated. Hopefully that helps bring some clarity on the complexity of auction eligibility behavior, and why it's more complex than it used to be.
Also, I'd like to go on the record of saying I really detest bid floors. The point of an auction is to identify what the market wants. If the auctioneer is artificially inflating the market, then it's no longer solely based on how the market actually values that auction. It doesn't help that bid floors are privately known only to Google, so yet another piece of the puzzle without transparency ;)
So hopefully that sheds a little light on what is occurring in your Google auctions. Keep on PPCing, but do so with your eyes wide open.