It's actually called bad programming logic.
When you have over an hour between blocks (596266 to 596267) and then over 30 minutes to the next block (596267 to 596268) it throws off calculations that are using a too small number of blocks to calculate the hashrate.

Crappy web programmers strike again.

144 blocks (1 day) is probably the bare minimum you can use to determine hashrate and that is cutting it closer then it should be.

The full 2016 blocks (2 weeks) between difficulty changes is the only real metric that matters.

-Dave