Your ‘ul’ element is set to have width of 1000px, and last child ‘li’ element is breaking on new line on some systems because there is not enough space for all elements in one line (one some systems due to font rendering, percentage calculations, etc..)
You could make ‘ul’ element 100% wide, and center inner ‘li’ elements.
.main_menu1 .wrapper {
width: 100%;
text-align: center;
}
.nav1 .main_menu1 ul li {
display: inline-block;
float: none;
}
That should fix what you would call a bug for screens larger than your wrapper max-width of 1000px. What it is; is actually a bad practice in regards to responsiveness of website to have child elements fit exactly in pixel in container; without use of percentage values, some other technique, or to be prepared if elements break on new lines. And you still have to fix issues with menu on smaller screens (below or near your wrapper max-width of 1000px). In less words; Menu is not exactly responsive.