Have a weird issue with project durations

Reducing Units Method
If I have a task that takes 4 days (set up using defaults) - If I reduce the persons units to that task to 50% - the duration increases to 8 days - no problem with this.

Calendar assignment method
However I have a separate calendar for PT time staff - who only work 4 hours.If I assign this task to someone using this calendar the duration does not increase (although the end date moves)

To a simple guy like me- I appear to be doing the same thing - as effectivley the hours per day are reduced from 8 to 4 in either scenario. - can anyone provide me with an explanation as to why the duration doesn't increase when I use the different calendar ?