What is 'conversions' (why it's not one number)?
Both Google Ads and Meta Ads report a metric literally labeled 'Conversions,' but it can mean different things depending on account settings and where you're looking. Google distinguishes 'all conversions' from the subset marked 'primary' (the ones bidding algorithms actually optimize toward), and Meta reports conversions broken out by individual action type, purchase, lead, add-to-cart, each counted separately. Comparing two 'conversions' figures without checking whether both are using the same definition and the same underlying action is a routine, easy-to-make error.
In Google Ads, every conversion action (a form fill, a purchase, a phone call, a newsletter signup) can independently be marked 'primary' or 'secondary.' The 'Conversions' column that bid strategies actually use to optimize only counts primary actions by default; 'All conversions' is a separate column that includes everything, primary and secondary combined. Two accounts, or two time periods in the same account, can show wildly different 'Conversions' totals purely because someone changed which actions were marked primary, with no actual change in on-site behavior. Meta's structure is different but has the same trap: its main conversions metric is really a sum or breakdown across whichever conversion events you've set up (purchase, lead, complete registration, add-to-cart), and the ads manager UI lets you pick which specific event to display as the headline number, so a 'Conversions' figure from one report and a 'Conversions' figure from another can silently be counting different events.
This is why comparing a 'good month' to a 'bad month,' or comparing performance across two campaigns, can produce a swing that has nothing to do with actual results: if the conversion action definition changed, or a different event was selected as primary, the number itself is measuring something different even though the column header hasn't changed. Before treating any conversions comparison as meaningful, check what specific action or event is being counted on both sides, and whether it's the primary/optimization-eligible definition or the broader 'all conversions' one. For someone managing an agency relationship without touching the platform directly, this is a fair, specific question to ask: 'which conversion action is this number counting, and is it the same one in both reports?'