Mar 13, 2019 - 11:47 AM
Jasper, you are looking at just one of the many metrics Facebook makes available. That isn't to say CTR isn't an important indicator, but everything else from your results (including the most important metrics) show that Facebook is in fact rewarding and optimizing on the better performing variation.
You are getting more impressions and clicks at a lower CPM and CPC from the cover on the left. Who cares if the CTR is slightly lower on that if you are getting better end results?
For whatever reason the audience that is responding to that image takes more impressions to get them to click, but they are also a cheaper audience to reach so in the end it works out better.
Your stated goal is to use the cover that performs better. I assume here you are trying to get the most clicks at the lowest CPC. In that case the one on the left is the winner so far.
I'll note that this might change if your objective is actually further down the funnel. For example, if you were actually running a conversion campaign Facebook would be optimizing on cost per purchase and your CPC or CPM would often be more expensive.
I often run conversion campaigns where the top performing ad or target group has a higher CPC, but lower cost per purchase.