Why did my percentage of ranking change so much in my Stack Overflow profile?

As you can see in the image below I’m currently in the “top 20% overall” group. A few days ago (may be up to December) it was showing something like 0.053% when my reputation was less than 1500, and it was changing while my reputation was increasing, but I am sure the numbers were like I said, 0.053%, and for the past few days (may be since the start of this year) it has changed to 20%.

So I am not getting clear why the percentage has changed so much though my reputation didn’t change such high. What are the reasons behind this?

Current rep with top 20 overall