Measuring brake rotors is critical to performing a pulsation free brake job. It doesn't matter if the rotors are new or old, they always need to be verified if they're within specification. Lateral runout or LRO is defined as an inaccuracy of the rotor's plates That does not allow them to rotate exactly in line with the flange. The rotor might have the same thickness, but the plates around the flange move up and down. In layman's terms, it's a wobble in the rotor. The wobble can be changed by machining the rotor to match the same plane using an on the car brake life. Replacing the rotor will not solve the LRO problem. LRO will not cause piston movement, but rather caliper body movement. This movement causes the brake pad to make contact with the high spot of the rotor and wear. This wear can cause changes in thickness to that area of the rotor.