When Obi-Wan and 'Darth Vader' meet in episode three, Obi-Wan adopts a very defensive stance. He wears down his brilliant but impassioned opponent in this way ultimately leading to Anakin's defeat. Furthermore Obi-Wan is known for his mastery of the defensive lightsaber fighting style.
In the older films, where Obi-Wan fights him again the scene pans out like so:
- Darth approaches Obi-Wan, red lightsaber 'drawn'. Starts to Smack Talk about how he is now a master.
- Obi-Wan responds, then draws his blue lightsaber and lunges towards Darth.
- Darth makes an easy riposte amidst further smack talk. Obi-wan continues to lead the combat striking first each time.
- Darth goes on the offensive, and Obi-Wan is forced onto the defensive.
- Obi-Wan gives his speech, spots Luke, gives a wry smile towards Vader and lets himself be killed/transcends into force-ghosthood.
You can see it in this video:
I'm not expecting Lucas to have foreseen the scene on Mustafar but, in-universe, is there an explanation? Is this just an old man leading when his strengths lay in defense?