as noted a few other times on this forum - most noticeably in the GMATPrep verbal folder, but in a couple places in the other verbal folders - pronoun ambiguity is not an absolute rule. in other words, the test will clearly tolerate a certain degree of pronoun ambiguity.
in general, we've found: the gmat will tolerate pronoun ambiguity when both of the following are satisfied: 1. the intended referent makes much more sense than do the other possible referents, and 2. the intended referent is PARALLEL TO THE PRONOUN, and the other possible referents are NOT parallel to the pronoun.