It’s a behaviour I’ve seen far too frequently, especially in JS-based slide presentation frameworks.
What’s likely happening is that behind the scenes they’re manipulating the browser’s history state when you scroll down to a new section. Upon scrolling to each section, they add the id of that section (e.g. #introduction, or #tutorial) to the tab’s history, probably using history.pushState(). That’s why when you do a ‘back’ operation, instead of coming back here to Lobsters, your browser instead pops one of those additional history items off of the history stack. It’s easier to see this happening with a browser that shows the complete url (including its /#tutorial part). They should’ve instead used history.replaceState() to avoid adding extraneous history items for each of the sections you scroll past.