Upvote:2
There can be a logic behind that time period.
Most countries require at least 6 month passport validity for entry. It doesn't apply for returning to USA as a citizen. You can enter USA without any issues upto expiry date of the passport and with some difficulties, even after expiry.
It can take maximum of upto 2-3 months for passport renewal. So, combined with both time period, they are making an assumption to renew your passport before that, so that your travel plans stays safe.