It isn’t at all logical.
Zip code areas are based on volume of mail, nothing to do with the number of eligible voters who live there. There are many office buildings that are a complete zip code on their own, but don’t have a single person living in the building. And we have a small town in Minnesota that does the processing for most coupons and mail-in rebates in the country – it is a very small town, about 1,000 houses – yet it has over 20 zip codes assigned to it.