Concrete precast plants require strict control over and adherence to the timing and sequence of operations. Variation for this research is divided into the variation in task starting time (the difference between the planned and the actual starting time) and the variation in task duration (the difference between the planned and the actual task duration). This study determined causes of variation in task starting time and duration of precast concrete slab production tasks and used STROBOSCOPE simulation techniques to demonstrate the penalties associated with not reducing variation, which are 1) overtime, 2) Work in Progress (WIP) increase, 3) cost overrun, and 4) labor productivity decrease. It was found that simply taking managerial actions, such as keeping workers waiting or busy, is insufficient for managing variation and effort should be put to reduce variations and make plan more reliable. The results could help prefabricators to understand the causes and penalties of variation, which is the starting point of attacking and reducing variations.