The leaderboard is there for you to compare yourself with the best anglers in the world, as well as people you follow. It is based on the total number of catches per species per calendar month, no matter size of each catch. Currently, you can only see species you’ve caught or that are common in your area.
If you are below someone on the leaderboard despite being on the same number of catches, it is because of the time of the catches. The person who reaches 5 catches fastest will be ranked above all other people with 5 catches.
If you are missing catches on the leaderboard, it is one of the two following reasons. Either the catch is missing a photo or the photo has not been approved due to inappropriate content or not including a fish in the photo.