Experiencing the same here.

2 theories:

1. Some of the people who have lb scores over 0.7 just have models that are highly overfit and their private scores will be lower

2. They genuinely have better models and found a clever way to account for the differences between the distributions of Train.csv and Test.csv as well as the imbalance in the data

it is just Good cross validation method solves a lil bit of this problem. for me my CV is 0.82 and LB is 0.72, it correlates well i guess