You are missing out on the additional information about you that they got when you added your partner, i.e. that you are in a long-term relationship. Married men are well-known to be lower-risk drivers than single men, presumably the insurer also knows that men in a relationship significant enough to buy insurance together are also lower risk.
Also, someone who hardly ever drives is very safe from the insurer's perspective. They may not be very skilled, but they also don't have many opportunities to get in accidents.
Even if that first part were true, we'd been together for a considerable time before she was interested enough in driving the car that we changed the insurance, so their risk analysis would still have been miles off for years.
But they can't base insurance premiums off of information that they don't know. They can't just take your word for it that you are in a relationship, because that is too easy to lie about. Buying insurance together is a clear indicator that would take significant effort to game.
But they can't base insurance premiums off of information that they don't know.
Of course. But as long as they can't form a sufficiently complete picture to make fair decisions -- that is, pricing based on actual risk -- discriminating on easier grounds that are correlated with risk but also happen to be incorrect in many cases isn't fair, so we make laws that stop them doing that.
Also, someone who hardly ever drives is very safe from the insurer's perspective. They may not be very skilled, but they also don't have many opportunities to get in accidents.