Upgrading PostgreSQL from 9.6.6 to 10.1

Question

I'm attempting to upgrade my local instance of postgres to 10.1 (from 9.6.6) on macOS. Using this gist as a guide, I've encountered a problem when running pg_upgrade. Here are the steps taken:

$ mv /usr/local/var/postgres /usr/local/var/postgres.old

$ mkdir /usr/local/share/postgresql.old
$ cp -r /usr/local/share/postgresql/timezone /usr/local/share/postgresql.old
$ cp -r /usr/local/share/postgresql/timezonesets /usr/local/share/postgresql.old

$ initdb /usr/local/var/postgres -E utf8

$ pg_upgrade \
  -d /usr/local/var/postgres.old/ \
  -D /usr/local/var/postgres/ \
  -b /usr/local/Cellar/[email protected]/9.6.6/bin \
  -B /usr/local/Cellar/postgresql/10.1/bin/

Performing Consistency Checks
-----------------------------
Checking cluster versions
Old cluster data and binary directories are from different major versions.
Failure, exiting

Prior to this attempted upgrade, 9.6.6 was running perfectly with /usr/local/var/postgres as its data directory. Why does pg_upgrade think the old data does not match the old binaries?

Edit: Here's the output requested by @jjanes:

$ /usr/local/Cellar/[email protected]/9.6.6/bin/pg_ctl start -D /usr/local/var/postgres.old
server starting
FATAL:  database files are incompatible with server
DETAIL:  The data directory was initialized by PostgreSQL version 9.5, which is not compatible with this version 9.6.6.

I had assumed 9.* was considered the same "major version", as per semver. Apparently not; upgrading from 9.5.10 worked perfectly.