Schedule variance is keeping track of the project progress by comparing the current money worth of it to the planned worth it should have. If at month 6 the earned value is closer to the planned value than you predicted than that should mean that you’ve made considerable progress in the project and are ahead of schedule. Some things that could affect the schedule variance are problems with the vendors, another project requires more attention if say a CAPA was initiated and you’re down in man power, some team members left the company and need to be replaced meanwhile other team members now have to take the extra load, or a piece of equipment needed for testing is broken, etc. In one case, the company I intern at was doing a sealing validation but it was discovered that the thermocouple of the sealer wasn’t properly working so it delayed the execution by a couple of days while the company delivered new thermocouples. This didn’t severely change the schedule variance but it’s an example of things that could happen and happen more often than not that could delay the project even by a couple of days.