When you installed WooCommerce in your WordPress site, a page called 'Cart' and 'Checkout' is automatically created under Pages with a shortcode inside to display the cart. These 2 pages pickup your default 'Select Layout'. So in your default Select Layout setting if you used Main + sidebar, these Cart and Checkout page will also have sidebar.

To remove this sidebar in your Cart and Checkout page, go to Pages > Cart/Checkout and select Full width layout at 'Select Layout' option.