why does website ranking differ by location?

Hi there, I was hoping someone could help with a local SEO question I have.

We track our ranking performance on SEMrush across 50 or so keywords. We have store locations in various cities across Canada, and have a position tracking campaign set-up for each city. This enables us to see how we rank in the cities we have stores. We also have a national position tracking campaign set-up, to see how we stack up at a national level.

I was wondering why we rank better in the cities we have stores, versus nationally? As an example, our visibility across tracked keywords is around 10% for each city we have a store in. For the same keywords at a national level, that visibility is much lower.

For various reasons, we don't make any mention of stores or the cities we have stores in on our website. We don't have any contact pages or the like that makes reference of our locations. How then does Google rank us higher in these cities? I'm not complaining of course, but I'd like to understand to help inform a future local SEO strategy.

The only thing I can think of is we have a Google MyBusiness location for each store location, pointing back to our website. Thus, if someone searches in a city we have a store, can Google pick-up on the fact we have a location near to that user (by way of us having a GMB location linked to our website) and subsequently rank our website higher in SERPS?

I'm struggling to think of any other way Google can figure out why it should rank our website higher in the locations we have stores if there's nothing that tells them that on our website.

Any help would be greatly appreciated!