Every now and then someone will comment on what they think is a strange shift pattern in their Magna - usually by ex Commodore owners (me too!).
The best way to discover if yours is correct is to check in the downloadable workshop manual: http://www.lisho.net/ - look in "Section 23A - Shift pattern". There is an excellent easy to understand diagram in that section with enough technical stuff to satisfy anyone.

Interesting! I've got the manual but haven't come across that graph before.

What is the relationship between output shaft rotation speed and road speed though?

Graph shows:
Each gear has a minimum speed at which it will change up, at 25% throttle or less.
At 25 to 80% throttle opening, The 2 to 3 and 3 to 4 change can occur over a wide range of speeds, I assume this is the learnt behaviour.
1 to 2 change is not included in the learnt speed.
Change points at 100% throttle are fixed.