Reported collected sales tax doesn't make sense

[PLIA]

My store shows $280.64 in net sales and $23.82 in collected sales tax (MI @6%).

If 280.64 was the taxable sales the tax would be $16.84.

If the gross sales was 280.64+23.82=$304.46, the tax would be $18.27

The Shopify reporting doesn't make sense. is it a bug?

Thanks,