The lightning strike distance estimate is presented as a range and there are actually five “steps” or “bins” of ranges: 0-5km, 5-10km, 10-20km, 20-35km, and >35km. In miles that works out to 0-3, 3-6, 6-12, 12-22, 22-32, and >32.
Correct - you won’t necessarily get an alert for every lightning strike.
Actually, it’s 15 (including “0”). Yes, the sensor itself provides a “statistical distance estimatation” to the “head of the storm”. If you’re curious, give the sensor’s tech sheet a read. It manages to be both incredibly detailed and incredibly vague about what those definitions actually mean!
Although this value can be one of 15 precise value (0, 5, 6, 8, 10, 12, 14, 17, 20, 24, 27, 31, 34, 37 & 40 km), we have found this value to be misleading in its precision. That’s why the presentation of the value is “binned” or “smoothed out” in our apps. We present the value as a range in an attempt to convey the uncertainty in the device.
Note, in addition to uncertainty in the distance estimation, the lightning sensor also has a variable “efficiency” in detecting strikes at all. This ranges from ~20% at the 40km range (missing roughly every 8 our of 10 strikes) to ~40% (missing roughly 6 out of 10) at the closer range.