Maybe he thought Obi-wan had learned the power of shrinking
Obi-Ant Man Kenobi!
Maybe Vader thought Kenobi had tried spinning just before striking him down? Vader does believe that to be a good trick!
He definitely already knew that
https://media.tenor.com/ZQo9-htHB18AAAAC/obi-wan-kenobi-spin.gif
