This depends on several factors, such as current block difficulty, block effort and block reward, which are the same for the whole Monero blockchain at the moment of mining. Also, pool distributes reward to miners only after it successfully mines a block. This process is probabilistic and sometimes may take few hours or more, so you will normally see XMR value lagging somewhat behind number of hashes. If you want to see real hash cost (CPH), use aggregated (daily) statistics.