add 16px to get correct width?

I am finding that I must add 16px to the width to obtain a correct width, and this applies to other browser-width plugins.

Here is a test: select the 1024px pre-set width from the plugin. That works correctly. Now use the text box to enter the width 1024. Do you see the browser width change?
For me I do -- why is that?

If I enter 1040px, there is no browser-width change, and again, this applies to other plugins. I have to use "1040" to get an accurate 1024px width, 1456 to get an accurate 1440px width, and so forth.