When a shade or blind has a standard width deduction, this means that the width measurement will be slightly smaller than the size listed. This is because when inside mounting, the shade should be slightly smaller than the window frame. If a product is the same size as the window frame, it may not fit inside and may prevent the product from raising or lowering smoothly.
The standard width deduction is usually between 1/4 inch to 1/2 inch and helps ensure that the product will fit and function correctly when inside mounted.
Please make sure to check the product specifications on each product's order page to note whether or not any deductions are included.