It seems to be switching to the mobile version of the nav bar anytime the active window width is less than 960px. The css being activated there is line 877 of flex.css
:
@media only screen and (max-width: 960px) {
.whmcscontainer {
margin-left: 20px;
margin-right: 20px;
}
Can either change that 960 or just get rid of it entirely depending on your needs.