I'm finding some inconsistencies with FB's ad display as mentioned by Polarbacon on 'hogging' of impressions by 1 ad in a campaign - Read post here).
After reading this, to get a solid idea of which ads can really do better for CTR and CR I have run some controlled experiments with Campaigns containing 4 identical ads - nothing else. The idea is to ensure that I don't miss out on the opportunity of a good ad because it is downplayed by FB in impressions volume.
Each time, 1 or 2 of the ads takes all of the impressions - similar to what Polarbacon found. Although he found that the highest CTR would win out and get all the impressions, I've seen something different at times, which seems counter-intuitive (maybe just an FB ad manager bug).
I have ads where they have a lower CTR, but get more of the volume of impressions (and as a consequence they also require higher bids to keep them going).
My only conclusion is that in some way FBs algorithm weighs 'impressions momentum' heavily - even to the point where sometimes an ad with better CTR can lose out on the opportunity to get 'impression hogging' status.
If others have seen this or have thoughts love to hear them.
The reason you might see a single ad with a lower ctr than some other ad in the campaign is because the current selected ad (by FB) has probably initially had the highest ctr, and must have already went down since the beggining, this is why it looks funny that the ad with the most clicks has lower ctr than other ads in the campaign.
Hope that clears it up a bit.