The stimulus tells us what will happen if the polls are NOT grossly inaccurate. But we do not know what will happen if the polls ARE grossly inaccurate. (”Unless the polls are grossly inaccurate, Slater will win” = If polls are NOT grossly inaccurate, Slater will win.)

We know that if the polls are not grossly inaccurate, McGuinness will be appointed. But this doesn’t mean his appointment requires the polls to not be grossly inaccurate. He might be appointed even if the polls aren’t a good indication of how the election will turn out.

If Slater does not win the election, we don’t know who will be appointed head of the planning commissions. It’s possible McGuinness could still be appointed, or perhaps someone besides McGuinness and Yerxes will be appointed.

We know that McGuinness doesn’t have both the quality of being an architect and being on the planning commission for at least 15 years, because we know he’s less qualified than Yerxes. But it’s possible McGuinness is an architect without 15 years of planning commission experience. Or he might have 15 years of planning commission experience, but without being an architect.

If the polls are a good indication of how the election will turn out, that means they’re not grossly inaccurate. This triggers the conditional chain in the stimulus, allowing us to know McGuinness will be appointed. And McGuinness is less qualified than Yerxes.