[求助]one comparison question

1.  The pay of senior executives increased in 1990 by a larger percentage than did the wages of other salaried workers.

2 .A wildlife expert predicts that the reintroduction of the caribou into northern Minnesota will fail if the density of the timber wolf population in that region is greater than one wolf for every 39 square miles.

My question is why 助词"DID" is not ommited in the first sentence, but 助词"is" is ommited in the second one.

助词尽量补出原则

GMAT是选最佳答案,如果补出了助动词的时态正确肯定选补出了的,但如果时态错误便选省略掉的 ^_^