Hey James , I totally know what you mean - been there myself. Here's the deal. Usually people interviewing you are managers that are just interviewing to fill an open spot (i.e. they are not professional recruiters or HR folks). So interviewing is not their strong suit. A lot of companies don't train for this or even think through the problem. So you might end up with someone who isn't focused on what they are trying to achieve.
That being said, if you have the core skills, talent and rep - then it just might be that the person is trying to figure out if you are a cultural fit (which could be what the conversation is about) or perhaps ensuring you know and are excited about the opportunity (which could be what the lecture is about). But, chances are that its just a person that is not very good at interviewing.

Suggestion is just to stay focused on how you can be a good interveiwee. Interviews are about the company filtering out whether you are a good fit or not for them, balanced with them selling you on why they are a good fit for you. Pay attention and you can actually feel that filter vs. sales dynamic switch if you are having a good interview. You can help steer the conversation to why you are a good fit for them, or interject a lecture with interesting questions which suggest the same. Just make sure when you walk out of that room, that they will have a favorable impression.
good luck - hope this helps.
Fred