you answered your own question.

If you look, and start booking a hypothetical trip, when you select award tickets to use, you can see that certain routes can definitely be booked in the method you describe and they only require 0.5 reward ticket (subject to the rules in the faq, like needing 4hr or less layover) while others require 1.0

If you build your own service for which there IS an alternate, scheduled route, the site (and the phone in system, AND the system even gate agents use) will take 1.0 award. If you build your own service for a trip where there is no alternate, scheduled serivce (thus you NEEDED to book it this way), as long as there is no overnight layover and the layover is under 4hrs, it will only count as 0.5 award

I had experience and another post on this earlier this year. we missed our flight mco-buf, then travelled mco-mdw-pit with a 3hr layover at mda. although our original mco-buf used only 0.5 reward ticket, the mco-mdw-buf segment that the GATE AGENT booked for us since we missed our fliht required and took two reward vouchers. the agent was mad at the compter and told us to board even without tickets, that she would fix things once airborne. the computer kept telling her 'insufficient funds for travel' even though she cancelled our mco-pit route. the insufficient funds had nothing to do with the PFC fee and everything to do with us not having 2 award voucher halves listed . imagine my surprise when I got back home and saw they took out an entire Round trip award voucher to fly me one way.

customer servce came to the rescue after many weeks and they did a one time courtesy 0.5 award ticket credit, with a stern lecture for watching out on my own (even though it was there mistake)