Cologne can evaporate around the sprayer in low-quality atomizer bottles. The way the atomizer is installed on the bottle dictates if a perfume evaporates. Some low-quality atomizers allow perfume evaporation. Airtight quality atomizers prevent perfume evaporation.
Over a long period of time, you can see your perfume levels dropping inside a bottle even without spraying any of it on yourself. This is called cologne evaporation. Even when you place the cap on the atomizer, you might still see diminishing perfume levels as a direct result of evaporation.