There are two common explanations, and both are good news.
The first is good behavior credit. Most county jails apply earned time or good conduct credits that reduce the actual days served. The math on that can shave weeks off a short sentence, especially if he has been compliant and incident-free. A three-month sentence with good time applied can land anywhere from two to two and a half months of actual time, depending on the facility's formula.
The second is overcrowding. County jails operate under population caps, and when they are running tight on bed space, facilities sometimes accelerate releases for low-risk inmates who are close to their release dates anyway. Denver County Jail, like most urban detention facilities, deals with this regularly. An inmate who was already near the end of a short sentence and had no disciplinary issues is exactly the profile that gets an early exit when space is the priority.
Either way, an earlier release date than expected is the best kind of surprise. Focus on making sure he has a solid plan in place for the first few days out, because landing on July 4th means some of the usual reentry resources may be operating on a holiday schedule.