For sure it's probably a human interference but I fail to see the logic in sentence below, why could they rule out interference if it appears on all places? "A

For sure it's probably a human interference but I fail to see the logic in sentence below, why could they rule out interference if it appears on all places? "According to Werthimer, the FAST telescope's receiver can look at 19 different places in the sky at once. Scientists are used to ruling out interference if it shows up in all 19, but if the interference only appears in one (as it did with all three of the supposedly "alien" signatures detected in this case) even experienced researchers can be led astray."