Network hashrate is calculated from the count of blocks found, there's no other way to determine it. But block frequency varies according to the luck. As we can't determine current luck, we have to take it into account always being 100 %. Sure this is not true, and so we are observing variations in network hashrate that don't exist in fact.
Certainly I don't want to say there aren't any variations. They are. But I can't tell how big they really are. Can someone go further and calculate the probable contribution of this effect to the total variation?

Miners' workers can tell their hashrate exactly.
The pools calculate their hashrates from shares of difficulty 1 or more. With ASICs comming, there'll be more of "more", and the accuracy will go down. Well only absolutely, maybe, and the relative accuracy can be maintained, as the hashrate increases also.
The whole network has no means to determine its total hashrate other than from the blocks found, as stated above. [edit:] With increasing difficulty, variations will increase and the accuracy will continue going down.