I should be honest up front that I probably have a tendency to over analyze sermons. If we're listening to someone share thoughts from God's word we should seek to get something out of it. We can probably all improve in that.
The thing I've noticed is, despite expository sermons being the mainstay of evangelical church services, most of them actually end up having a topical focus anyway.
Let's define a few terms: Expository (to me) means taking a passage of Scripture and explaining what it says, what it means, and how we should live because of it.
Topical is taking a topic and explaining what the Bible says about it and how we should live because of it.
I personally am perfectly okay with either type of sermon, but often the expository sermon gets lifted up as "better" because it sticks to Scripture and there's less chance of coming up with a message that isn't quite true.
What I notice though, is that many expository sermons end up being topical. The speaker reads the passage, makes a few comments on it in general, and then picks a topic from it and builds a whole sermon around that topic.
Nothing wrong with it, just an observation.