How is state and local tax deduction calculated

I live in a high-tax state. Last year my SALT deduction was about $15K, which I assume was the amount that my spouse & I paid in "State income taxes" (W-2 line 17) plus property taxes paid -- correct?

I also know that the new tax rule caps SALT deduction at $10K.

So I am trying to figure out why even though this year we spent almost $13K in State Income Taxes + property taxes, why TurboTax is showing SALT tax total of about $8K and not an even $10K (the cap).