The tendency of a very good writeup
to create shear
forces on other decent but unremarkable writeups in the node
If the second (or later) writeup exhibits sufficiently great nodecraft, the first writeup may be forced down as on a pivot because it looks bad by comparison. On the other hand, one amazing writeup in a given node can transmit vote shear to other writeups in the same node. One writeup with a reputation of over 250 (as of November 2000) can attract reputations of over 50 to what are essentially "me too, damn straight" writeups -- decent, but unremarkable.
Moral of the story: It's probably a good idea to vote on each writeup individually.