This can occur for several reasons. Each trade is scheduled days or weeks in advance of the actual work. Occasionally, one trade completes its work a bit ahead of schedule. Sometimes their job takes longer than anticipated. When either occurs, schedules have to be adjusted.
Progress also pauses while the home awaits various inspections.
This is part of the normal sequence and may occur at several points in every home. Finally, keep in mind that a quality builder would rather wait a few days for the best craftsman or product than hurry the process. Most homebuyers will quickly forget moving in ahead of schedule, but not forget poor workmanship. A good builder will respect and appreciate your interest and attachment to your new home. Your input should always be welcome.