I’m not sure why the data would change. I can have a blog post in Markdown that spans across multiple apps, same with other standard data formats like MP4 and PNG which do not need to change when porting from one site to another.
The post body is just one property of a JSON-LD document in ActivityPub. The rest of it is metadata, relations, etc, that different apps may present ever so slightly differently.
It's more like an API with multiple implementations.
(Even just JSON serialization differences will pop up. For example, PHP always likes to escape forward slashes.)
