This can happen when you don’t plot accurately near a junction – you will have clicked somewhere on the side street, which can also result in the wrong turn arrows in the directions.
To avoid this happening, it is best to click away from junctions or zoom in closer when plotting.
We have a number of detailed guides giving instructions on how to use many of the features of plotaroute.com.
We have a number of video tutorials demonstrating how to use key features on the site.