The issue is because your ‘Nav Announcement’ element is not sticky with the nav element. If you check that gap is the same size as your nav announcement. My suggestion would be to make the announcement to stick with the nav as well.
I don’t suppose there’s any other way as I would rather the ‘announcement bar’ didn’t join the nav all the way down the page… I could reduce opacity on scroll i guess but then there’d be a big ‘blank’ space at the top of the nav. hmmm…