Explanation for discrepancy in U.S. weighted average tariff data

[Bryan Riley]

Can someone suggest how I can understand how the United States Product AHS Weighted Average By Country and Region is calculated?

According to WITS the rate is 13.78 percent in 2019 and 1.52 percent in 2020. 

But when I use the U.S. government's import data, I get a weighted average tariff rate of 2.6 percent in 2019 and 2.8 percent in 2020. That is calculated by dividing duties by total import value. 

I would like to compare U.S. to foreign tariffs in 2020. But I don't know if I should use the WITS 1.52 percent rate or the USA 2.8 percent rate. More generally, I would like to better understand what goes into the WITS AHS calculation. 

Thank you.