The facilities and traits affect the random selection process that determines which system gets hit by a generated event. The settings.txt stuff affects the chance of an event being generated in a turn.
The facilities and traits have no affect on whether an event is generated or not. They only come into play when the game selects a random target for the event. This is because the game first randomly determines if an event is to occur on a given turn, based on the Settings.txt lines. Then, it picks the type of event randomly based on severity settings and what is in the Events.txt file. Finally, it selects a target for the event. This is where the facilities and traits come into play: they can make it more or less likely that an object in a given system will be chosen for the event to hit it. But, the event has already been generated, and will hit something regardless of the facility/trait modifiers (if they add up to 100% event reduction and are in all systems, the event will still occur anyways). Unless, of course, there is no valid target for the event, such as a ship bomb when there are 0 ships in the game. I do not know what happens then; the game either cans the event for that turn or it picks a new event type.
The combat sensor is not handled in the same way at all, because vastly different functions are at work for it.
Basically, the (for example) "base chance" of 30% of an event hitting a system is wrong. It is just a 30% chance that an event will be selected to occur on a given turn. The "base chance" of an event hitting any particular system is 1 / total number of systems (x 100% if you want a percent value instead of a decimal
).