Ya know, I havent looked into it. I also was a little confused about the "ranked experience" - its weird to have 2014-2015 as your source of experience, right? I'm glad others picked up on it.
Now, the explanation that I'm hoping for is that its a failure by the moderator to update his site - when the season changed, they didnt update the category. Then the statistic would be more useful and relevant, especially if they meant "The season that occurs between 2015-2016."
It would also explain why many champions have a dropoff in winrate at 125+ games played. Youd expect them to be the most practiced, right? Its weird that some champions winrates are highest at 15-50 games played but significantly lower at 125+. But if it were only took the data from this season, it makes sense - the ones at 15-50 games are still climbing, while the ones at 125+ have climbed to the highest they can get.
This conjecture isnt exactly out of nowhere, but theres nothing proving it. My personal thoughts - the site is run automatically. It gathers the data from Riot's API, and there are programs in place to sort the data into the categories we see. In the F.A.Q.s section, the creator/moderator links to his reddit, /u/joeldo. It seems like he had very high activity over a year ago, but has 1 post (dated 9 months ago) since then.
I figure that he was done with his project and moved onto new things. The tag still reads 2014-2015 ranked experience, since thats when he was still active, but the actual numbers are from this season.
Or maybe its wishful thinking.