As you have mentioned, Schedule variance specifies project whether how much ahead or behind the schedule. Some factors that can affect the project schedule variance are unexpected changes to the design of the project. These changes bring the new request on the project which causes the timeline of the project to change most likely increases extra days. Some of those changes that affect the schedule variance are changing in the delivery schedule of parts, work is not progressing based on the planned schedule, and any sudden requirement changes. In order to avoid these changes, have regular communication with the vendors, and team members. That way, real solution can be taken care of in quick time.