The rebels don't care about the borders that the Empire has set up. They'll operate wherever they can, and the ISB was content to belive certain events weren't connected or inter-agency communication wasn't necessary bc they happened in different sectors.
The Empire isn't operating in a way that allows them to identify and eliminate rebel threats that are working across sector lines/borders. Dedra is trying to get the other sector chief to share information, but he is reluctant to share it, and so the Empire is having a much harder time piecing together Luthen's plans.
Every sector likely has its own "local government" which includes Policing actions, anti-terrorism units, politicians etc. This adds complexity to their sector-to-sector communication. On top of this these sectors do not help each other as we can see from your scenes; there's significant political in-fighting. I believe in some scene(s) Diedra asks for information on X, and the immediate response she gets is a "file a request with Y". Which obviously bogs her down in bureaucracy.
Basically, each sector is inside their own informational vacuum.
The rebels exploit this by specifically working across sector lines, because no/little information is shared each sector may only gain some insight into rebel activity. But they do not realize that it is much more organized.
And this sector-thing isn't a Star Wars fantasy neither. Russian units tend to attack Ukrainian defenders specifically in between sectors (that are divided between brigades) on the frontlines. Why? Because when they do so the Ukrainians have a much harder time organizing themselves because they don't have as good communication with their sister-brigades as they do "internally".
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