Why does the tax rate calculate differently when I pull the "sales by item report"?

Good Morning,

Our set tax rate is 6.5% for all our items. However, when we pull the "sales by item report" and calculate the tax rate it fluctuates.

Example: 2 items bought = $1.90 (Gross Sales) Tax = $0.12, Total = $2.02

When we calculate the tax rate for the sale it reflects a lower tax rate.

Example: $0.12 (Taxes on sale) / $1.90 (Gross Sales) = 6.316% 

Why are we not ending up with a 6.5%?