Cooling doesn't always mean running a compressor — sometimes the cheapest ton of cooling is the one you never mechanically make at all.
A mechanical cooling coil doesn't care what's happening outside. Whether the outdoor air is 100°F or 50°F, a compressor-driven refrigeration cycle has to run to pull heat out of the supply airstream, and every hour it runs costs real compressor energy. But an air handling unit has a second, much cheaper way to cool a building: when outdoor air is already at or below the condition the space needs, simply bringing in more of it does the job — no refrigeration cycle required. That's an air-side economizer, and getting the switch between the two modes right (or wrong) is one of the biggest levers on an HVAC system's energy bill.