"Terrific absorption has pushed occupancy upward to highs for this economic cycle," say Greg Willett, chief economist for RealPage Inc., a provider of property management software and services. That might seem strange. The occupancy rate in the U.S. has already been high for a very long time. Economists and apartment experts have been expecting more apartment units to become vacant for years, as more developers open new buildings. But demand from renters remain strong, despite growing worries that the U.S. economy may be weakening.