I'm not exactly sure why it does this, but my guess is that it wants to show you that there are "plenty of options", and the fewer options there are in your zip code, the farther out it zooms. So, searching a rural zip code where there are fewer sitters would zoom out farther than a densely populated area with lots of sitters nearby. That's just my guess.
One thing you can try as a workaround is zooming in the map until it resembles the search area you want to explore, then clicking the numbers icons closest to you. Make sure the checkbox above the map that says "Update when I move the map" is checked. I hope that's helpful.
Because I have done this🤣, make sure you are not logged in....took me awhile to remember that.