That’s a reasonable question for immigration, especially if they think you might try to work illegally (which, as a native English speaker, you could do easily). Canada can be quite hard on Americans for this, and we even have a special arrangement where the eSTAs and eTAs don’t apply between us for US and Canadian citizens. We have nearly complete freedom of movement.
My wife is usually quiet when we do immigration because I carry the passports and know the process (a bit silly, since she’s been through it almost as many times as I, but they need to be in one place and I carry that bag), but at the land border I’ve had to wake her up to get her to talk. She asked why one time. I said, you have a US passport, we have different last names, they have to make sure I’m not a human trafficker with a fake passport for you.
Then she got Global Entry and that issue went away. They know both of us.
My wife is usually quiet when we do immigration because I carry the passports and know the process (a bit silly, since she’s been through it almost as many times as I, but they need to be in one place and I carry that bag), but at the land border I’ve had to wake her up to get her to talk. She asked why one time. I said, you have a US passport, we have different last names, they have to make sure I’m not a human trafficker with a fake passport for you.
Then she got Global Entry and that issue went away. They know both of us.