Why would my health insurance provider call me to spontaneously offer a lower rate?

Asking anonymously for privacy.

I have had both basic and extended health insurance at company "A", which is now a sub-brand of company "B".

Today, I was called by someone identifying as from company B. They started by thanking me for my fidelity, since I’ve been with them for my entire life, and then asking me if I wanted to switch my basic coverage from company A to company B, to save a small amount of money. According to them, there would be "no difference" for me, only slightly lower fees.

Why would they do this?

I think their claim sounds believable, since this is the law-mandated basic coverage. On the other hand, I fail to see what’s in it for them. It wouldn’t be even to simplify things administratively, as it’s probably simpler for them to handle both the basic and extended coverage under the same brand.

I’m very wary of touching my health insurance, because I have several lifelong conditions requiring an expensive treatment; the treatment cost alone (covered by the basic, law-mandated plan) barely lets them break even on my fees, before any other medical expenses come into account.

Is this a backhanded trick that will somehow help them make my case more profitable for them? Or are they just afraid I’ll switch to a less expensive provider for the basic coverage?