Most companies create an annual business plan, outlining revenue targets for each month. If a target is consistently met or exceeded, the company will achieve their profit budget. In reality, their actual sales performance is often very erratic. Some months they achieve their target, and some months they fall short. This wreaks havoc for forecasting, and business planning purposes.
Inconsistent sales performance is a symptom of the challenge above. Companies that struggle to attract, develop and keep good sales people experience huge variability in performance from month to month, and from person to person. This is also reflected in performance on an individual level. The sales people themselves go from hot to cold, and often fall short of their numbers.