I believe it's more of a line of sight kind of thing. If you are in a car, it's sitting in the cradle on the dash. If you are walking, nothing is blocking its line of sight.

Other GPS units that may lock in better without line of sight are likely pushing more power to the antenna.

If I put it near a window, it'll lock in within a minute. If I pull it away from the window it sometimes loses signal within a few minutes.