The hash rate of the network is in fact NOT the hash rate of the network. It is an estimation of what the hash rate of the network should be in order to keep blocks coming at an average of 1 every 4 minutes. After looking at that hash rate over days, it seems to have a fair amount of hysteresis, so will wobble around as actual network hash rate (which is unknown at any point in time) varies. If a pool has the vast majority of the network hash rate, this will cause that pool's hash rate (which it knows) to sometimes be above the estimated (from difficulty, itself a function of recent past block times) network hash rate.

If you try any other pool, this effect will be the same, but you will not notice it if the pool is a lot smaller (like mine, for instance, hovering at 4 or 5 kH/s currently), because the values are so different.

By the way, this effect, if what I described is real, causes the pool's "luck" factor to give me really lucky blocks (eg, my pool found a block at 1469% luck recently) when that hysteresis causes the difficulty to go keep going up when the actual network hash rate starts going down. So it's all a bit inexact/approximate.

(OK, the 1469% is mostly down to actual luck, but I'm pretty sure the hysteresis effect also plays a role).