The difference between Page Node ID and Page ID is that they are two completely different objects. The PageNodes also have ID's, but they are different from PageIDs. Every Page has a PageID, but PageNodes are used for the organization - just like a hierarchical taxonomies and you can consider them as a container for the Pages and PageIDs.
You can get a PageNode by its ID, that is why your code works when you call the ID of a Page Node.
For example if we look at this code:
myPageNode = App.WorkWith().Pages().LocatedIn(PageLocation.Frontend).Where(p => p.Parent.Id == SiteInitializer.FrontendRootNodeId).Get().ToList();
You will get all pages, which have FrontEndRootNode as their parent.
I hope the above information helps you understand the differences between the PageNodeID and the PageID.
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