The thing is that if your conversion happens within the conversion window, which normally is set to 30 days, the conversion will give cred to the campaign involved in the conversion: no matter where in the funnel it happens. Basically if someone makes a purchase on your sita and has clicked on a google ad within the past 30 days- that conversion will show in your google ads account. You click on a google ad- dont buy. 3 days later you click on a facebook ad and buy. You will see 1 conversion in your google ads account, but also one in your facebook account. Even though there were only 1 conversion in total.