TLDR: Facebook says, " Total number of the organic reach and paid reach may not always be equal to the sum of their values." You can find the explanation below or click here to go directly to the page with this description 👇
For those who have more time, it will be hard to explain but we will definitely give it a try 😅
In the screenshot below, you see a content card of one of our posts from our Facebook page reports. And as you can see in the screenshot, we calculate the engagement rate by this formula: lifetime engaged users/ lifetime post reach * 100 and it’s the same as Facebook’s calculation.
We expect to see the reach value on the card (20.140) as lifetime post total reach in the formula, but here we see a different lifetime post total reach (20.084), instead of 20.140.
And now, let's look at the lifetime post total reach value on our FB report for the same post. As you can see in the attached screenshot, the sum of lifetime post paid reach and lifetime post organic reach is equal to 20.140 (same as the reach value on the post’s card), however, Facebook shows lifetime post total reach as 20.084 (same as the lifetime post total reach value in the formula).
We calculate the engagement rate according to the data that we provide from Facebook’s API and accordingly, we calculated the ER with this formula:
140 / 20.084 * 100 = 0.697
Once again, when we checked the engagement rate formula of the same post right from Facebook's interface, we also got the same result, which is 0.697. Please see the attachment below.
That means it’s completely normal that the reach value on your post card is not the same as the lifetime post total reach in the engagement rate formula.
Facebook explains the reason for the difference as “total numbers of organic reach and paid reach may not be exactly equal to the sum of two always”. If you are seeing two different reach values on your post’s card, it’s because of this inequality here ✍️