“Employment statistics indicate that the percentage of workers who found jobs in the fall quarter is lower than the percentage in the spring”
and
“Employment statistics indicate that the percentage of workers who found jobs in the fall quarter is lower than the equivalent percentage in the spring”
would in fact both be correct. So I would be surprised if they were both actual options on a real college board question. But maybe the verbal experts here can enlighten us…
In reading the original post, we are given two possible answers, the one the OP says is marked correct and the one the OP says is what he feels could also be correct but is not. The one that is not correct says "workers THAT . . . " where the one that is correct says "workers WHO . . . ". The one that is correct also throws in the word “equivalent” and that word is not in the answer that is incorrect.
I think the WHO vs. THAT is the crux of the issue that made one answer wrong and one answer right.