You probably had what's called a false-negative. In other words, the MRI didn't show anything wrong when there really was something torn. There are several things that could cause this.
First, the level of MRI technology makes a difference. Low field scanners are used in the doctor's office for a quick look. They are less expensive and immediately available, but not as accurate as the more traditional MRI equipment.
A false-positive can also occur if the patient moves during the MRI. It's possible to strain a ligament and then later re-injure the knee causing a tear to occur. In such a case, the MRI taken at the time of the first injury would be negative. A true positive might be found if a second MRI instead of arthroscopy was done after the second injury.
In your case you went from an MRI to arthroscopy. This is the normal sequence of tests when trying to diagnose an injury.