Arthroscopic examination of the knee is done with a needle inserted into the joint. There's a tiny TV camera on the end of the scope giving the doctor a view inside the joint. The test is usually very accurate.
The arthroscopic exam may be considered "negative" (normal) if and when the tear is small and remains flat against the rest of the meniscus. This is called a nondisplaced tear.
Meniscal tears are graded based on their location, direction of tear, and depth of injury. A horizontal partial-thickness tear may remain undetected when nondisplaced.
It's also the case that mild tears can become severe tears with re-injury. If your arthroscopic surgery was done before the tear was complete then a false-negative test may be obtained. This means the test was negative for a meniscal tear when there was a tear after all.