A lightning detector has limits. Basically a too close storm will send it in disturber mode. The energy is too high and it has to protect itself by turning down it sensitivity and there is a point it goes ‘deaf’.
We see this also with more sophisticated detectors like the ones from Blitzortung (another detector further away will take over and it is the network as a whole that manages) The one in the Tempest is meant as an early warning and not a perfect measuring system. Maybe the closest one you looked at was far enough as to avoid the ‘deaf’ situation.
Also the surrounding has influence. If you have a heavy spammer (airco, induction cooking plate … bad old relais etc) your detector is already in a disturber mode by ‘default’ and hears less. And the spammer can be your neighbourhood and not especially something you have.