Two names, one number
Each name you type is resolved to a single latitude and longitude, and the great-circle distance between those two coordinates is reported. That is the whole operation. It is the crow-flies distance — no roads, no flight corridors, no ferries — so treat it as the shortest distance that could theoretically exist between the two places, which any real journey will exceed.
A city is not a point
The interesting weakness here is not the arithmetic, it is the assumption underneath it. Turning “Paris” into one coordinate means picking a representative spot — usually a civic centre or the centroid of the built-up area. For two cities a thousand kilometres apart, that choice is noise. For two suburbs twelve kilometres apart, the choice of centre point can be a meaningful share of the answer.
Ambiguity compounds it. There are dozens of Springfields, two well-known Cambridges, and a Trinidad in at least five countries. Adding a region or country to the search — “Cambridge, Massachusetts” rather than “Cambridge” — removes the guesswork, and the resolved place name is always shown back to you so you can confirm it landed where you meant.
When the endpoints matter more than the labels — a specific trailhead, a particular gate, a corner of a field — clicking the exact spots in measure distance avoids the whole problem.