This bit always struck me as a rush job.
For a month they’d been working with Wilson’s draft from the Committee of Detail. That draft, if you recall, said amendments would be proposed by 2/3 of the state legislatures, and ratified by a national convention.
Then on September 10, there was a lot of argument over it, culminating in Wilson’s new version saying amendments would be proposed by 2/3 of the states, and ratified by 3/4 of the states. The convention idea had vanished along the way.
At which point Madison jumped in with this completely different last-minute substitute motion. The convention shelved Wilson’s indefinitely, voted on Madison’s, and passed it 9 to 1. Madison had re-introduced ratification by convention, but now state control of the process had disappeared!
Just days away from September 15, the last day of business for the convention, they now…
…well, we’ll see what happened next on the next page!