Top setting being altered when scrolling

I have Top set to 145 px in Settings.

On some pages this works fine but on other pages when scrolling down the page and the sidebar reaches its supposed fixed position, the CSS value for top is initially being set at 145 px but on the next page scroll it gets changed to 133 px. This is the output from Chrome Developer Tools for the sidebar.

element.style {
    will-change: transform;
    transform: translateZ(0px);
    width: 307px;
    position: fixed;
    top: 133px;
    left: 381.5px;
    z-index: 100;
}

This is the CSS for a page that displays OK.

element.style {
    will-change: transform;
    transform: translateZ(0px);
    width: 307px;
    position: fixed;
    top: 145px;
    left: 381.5px;
    z-index: 100;
}

Why is the plugin changing ‘top’ to a value less than it should be?