Winrates, Lanes and Numbers - A Puzzle

I noticed something interesting going through a few third party websites (Lolalytics, League of Graphs, Lolstarz and Champion.gg). I'll use Champion.gg given that it's the one sited most frequently on the forums (but the data is consistent across all four websites - even before the Riot API rework - excluding Lolstarz given that they are still in the process of updating for the API). Using yesterday's data, setting 51% winrate or higher as the standard, and focussing on those champions usually played in the respective lane roles:

• Top lane: 22/55 (40% of the main champions played in this role in Plat+ are 51% or higher in winrate)
• Jungle: 17/45 (37% of the main champions played in this role in Plat+ are 51% or higher in winrate)
• Mid lane: 20/45 (44% of the main champions played in this role in Plat+ are 51% or higher in winrate)
• Bot carry: 6/17 (35% of the main champions played in this role in Plat+ are 51% or higher in winrate)
• Support: 6/28 (21% of the main champions played in this role in Plat+ are 51% or higher in winrate)

Two things seem to stand out. First, there is a lack of variety of champs in botlane - compared with the other roles - I think that's something that has been brought up before. Second, and this is the thing which intrigues me - it seems that the botlane carry players and, especially, the support players really have few champs that seem to be working in that lane.

Any ideas on what is happening here? And, does anyone have any ideas on why it is happening?