Go back to the park and walk around and match what you see in the field to the map. Vegetation density is a subjective judgment of the mapper.
Vegetation also changes with the seasons.
Every map will be slightly different. Part of the skill of the sport is to quickly calibrate youself to the standard of the map and the mapper. Next time you are at an event, ask someone for some advice. Stay until the end of the event and follow someone picking up controls. You can then study the map and terrain at a more lesiurely pace and see how they interperet the map.