It seems likely that Census simply misclassifies a large chunk of online sales.

似乎普查局只是将一大部分网上销售错误分类了。

youdao

If an employer intentionally misclassifies a worker, the employer is liable for the full federal income tax that should have been withheld, plus 100% of all Social Security and Medicare taxes!

FORBES: Worry Over Worker Status Isn't Crying Wolf