Actually wavelength λ corresponds to 360 degrees , right? then why λ/2...
Don't expect Smith chart to work like an Argand diagram.
Consider a short circuit on an electrically short transmission line. This will have reflection coefficient of -1, which will be plotted on the far left side of Smith chart. If I lengthen transmission line by 1/4 wavelength in front of short circuit, it will have a reflection coefficient of +1 (i.e. it will look like an open circuit), which will be plotted on the far right hand side of Smith chart. Extending transmission line 1/4 wavelength moved us halfway around Smith chart.
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