Not being an expert, here are my thoughts.
- PR is something per page. Thus it can be possible that a page with a certain post has a higher PR than the home page.
- I think that Google uses PR to indicate authority on a certain topic. A post on a specific topic may therefore have a higher PR than a general page like the home page.
- Google probably uses an algorithm that defines how relevant incoming and outgoing links are to a certain page for a certain topic (recent social media links may influence this as well) and therefore give your post page a higher PR than the home page.