The scheduled variance could be ahead of the schedule if the planning of the project is very safe in terms of the number of days allocated for a particular stage is more than actually required.
Also sometimes people work overtime so that helps in finishing the tasks earlier than expected as the plan is made considering people work within the working hours and overtime is never considered.
The scheduled variance could be behind schedule due to various factors as the employee is on sick, climate is not suitable, the vendor has not dispatched the part required, the part send is damaged, etc.
I think the project manager has to know the complexity of the project he is working on and study which stages will take time and which would not and plan the project accordingly.