Unless things have changed, when I took my drivinf test I was told the following:
Always try and get the first test of the day.
Otherwise, if the tester happened to have the world's most perfect driver before you, then your minor mistakes will show up more. Of course, it would work to your advantage if the previous one was useless but better to have a "clean slate"
Secondly, testors were expected to fail a certain percentage of drivers. So if he had passed loads of people that week and needed to get his percentage down, then he might fail you for minor things when on another day you would have passed.
Don't let it get you down. You'll pass next time.
Statistically speaking, drivers who failed their first test are less likely to have an accident.