It seems that the network rate updates a lot slower. When charnacoin.com was at 1MH/sec the network was still showing around 200 KH/s. Same thing when the hash rate is dropping. Network rate follows slowly. Maybe it's a configuration option in the pool software.
 
Edit. Doesn't seem like it either. Where is the pool running the missing over 250 kh/s?

Edit2. http://chrc.cryptopool.me/#pool_blocks show that they have found almost all blocks recently. Yet their hash rate is around 50 kh/s versus network at 310 kh/s.