My guess is that this will happen when the real time network hashrate spikes up. The Network Hashrate as reported by the blockchain and other pools is based on a 4 hour average. Therefore, it will have a lag. The Pool Hashrate is probably based on a shorter time period. If the hashrate of the pool is higher than the network, that means that a large amount hash power is being added. Over time the 4 hour average Network hash rate will increase.
Large changes in network hashrate are probably due to the difficulty changes and miners looking for the most profitable coins.
