What the watch can do on which device
On iPhone the watch measures only while the app is in the foreground and the screen stays awake. The iOS bundle carries no background mode for location — it was deliberately removed, and what is still readable about it in the file sits in comments and has no effect.
The screen lock that keeps the display awake is requested only while the page is visible. In the web view on iPhone that interface does not exist at all; there the app says it is unsupported rather than claiming otherwise.
On Android it is different: a real foreground service with a location purpose runs there, declared in the bundle and implemented as a class of its own. This split between the two devices is named here and not blurred.
The radius, and what actually makes it smaller
Four buttons preset twenty-five, fifty, seventy-five and one hundred metres. Any number from five to three hundred metres can be typed in.
When the measured distance exceeds the radius, the app names both correct answers: raise the radius, or take in chain. Taking in chain really does shrink the circle; raising the radius only makes the warning quieter. The app recommends neither — the choice depends on the room in the bay, on the neighbours and on the wind expected, and the app knows none of that.
Why that is a difference
From outside, an alarm on a growing distance cannot be told apart from a dragging anchor. The numbers alone do not separate them; only the chain laid out does. So the watch computes the chain ratio alongside and shows it before and after.