Q: Why do some of my imported WordPress posts from my old site have different IDs on my new site?
A: You added ACF (advanced custom fields) which also use page IDs. On your live site you continued to make posts while working on your development. When you imported your posts the post IDs increment as they were already taken (by ACF components). As a result you will need to look out for old post URLs that now map to attachments or incorrect posts – these will look like 301 redirects and not 404’s.
This should only affect you if your permalink structure finishes with %post_id% instead of %postname% as it’s the ID which has incremented.