There is absolutely no way 645 is twice as hard as (6x,4). 645 is if anything slightly easier and if you look at little more closely at your data, you will see better jugglers tend to have longer runs of 645 than (6x,4). You also have data such as lucasgabd, where it looks like he just hasn't really spend time on 645, skewing the data.
| lucasgabd | 116 | 18 |