Can't you just put that down to the extra distance you need to run round trees?
Any chance of a link to the map or even GPS traces - I'd love to see them - and use them to show youngsters how it is possible to run straight.
The GPS you should be able to see on the globe next to the session. Slightly doubtful GPS vs course as Spongey and Gaute both ran under the course distance but to me my GPS looks pretty straight.