The total stake of a pool consists of ADA coins of the pool operator (pledge) and delegated ADA coins. The size of the pool determines the chances of the pool to become a slot leader and create a new block in the assigned slots. However, the chance is not purely proportional to pool size. For the sake of protocol security, an element of chance comes into play. Let's say that, based on a calculation that takes into account the proportional size of pools, a given pool has 40 slots in an epoch in which it can create a new block. This is the expected number. However, due to randomness, it may happen that a pool is assigned a larger or smaller number of slots. In the end, the pool can create either 45 or 35 blocks. Pool performance is calculated as the number of blocks that a pool has produced in a given epoch, divided by the expected number of blocks. When the pool has created 45 blocks, its performance is 112.5%. When it has only created 35 blocks, its performance is 87.5%, The pool reward per epoch reflects the performance of the pool, so it can be higher or lower. It is important to know that it does not matter whether a pool is currently above or below 100% of the expected performance. Within a longer period of time the randomness is fair so the performance is expected to be ~100% provided that the pool creates blocks in all assigned slots.
