Wind, shadows,ect are beyond anyone's control but all play a factor. Cover takes a beating after that many dogs run the same test. The "tough series" may not be so hard. The one time I ran, the tough series was first. There is a lot of luck involved for most dogs. There is also an elite group of dogs that pass regardless of conditions. These dogs are truly exceptional.
There are a LOT of dogs with one Grand pass for a reason.