It can not be difficulty is calculated based on just 2 last blocks, that would be foolish. All cryptocoins use at least few hundred blocks for PoW and PoS difficulty retargeting.
And I totally doubt it took 467226 blocks for the first ever occurence of PoS block with timestamp in future.
Well, tranz' fix addresses just that by setting it to zero when it gets negative.
Since it also does some other calculations, it does not yet have a huge impact when it's only slightly negative.