Vander Els Partners
Because the map has no single rank. It has a different one from every point a customer stands on, and the one you keep checking is the point where you always look best.
When you search your own service from your desk, Google measures the distance from your desk to your listing. It is zero. You come up first, or close to it, and it feels like the problem is solved.
A customer twelve minutes away is running a different search. Google measures the distance from her kitchen to every business in the category, weighs that against reviews, the category on the listing, the content on the website and the rest, and draws a new top three. Your competitor two towns over may be closer to her than you are, and if his listing is as good as yours, he wins that kitchen.
So a company can hold the top spot at its own address and be invisible across half the area it drives to. Both are true at the same time. Neither is the number.
What the real number looks like
Lay a grid of points across the area you actually send trucks to. Run the same search from every point. Colour each point by where you came up. What you get is a map, not a rank: solid near the office, thinning as the distance grows, and usually a hole or two where a competitor sits between you and a neighbourhood you want.
Run it again next month from the same points. Now a change is a change, not a different vantage point on a better day.
What moves it
Distance is fixed. Everything else is not. Reviews, the categories on the listing, photos, posts, the pages on the site that name the services and the towns, and the citations that tell Google the business is real and where it works. The work is unglamorous and it is slow. Movement across a grid usually takes months, and anyone promising the whole map in thirty days is describing paid ads under another name.
The short version
Stop checking from your desk. Check from where the customer is standing, from many places, on a schedule. Then you will know where you disappear, and you will know when it changes.