Nav Bar sits correctly in Preview, but not once Published

OK so, I feel like I’ve exhausted my knowledge here, but maybe I’m missing something obvious to one of you.

My Nav Bar button, once clicked - results in a drop down of the navigation options. On my preview, I have them perfectly lined up next to the button, with a little gap between.

For some reason, once published - the options are floating a little below and to the right, as if they’re being anchored to some other point. I can’t for the life of me see any logic for why this should be happening.

Any help would be very much appreciated.

Here is my site Read-Only: