Alarm clock with snooze interval of 9

Problem :

Why is it very common to have a 9 minute snooze interval on alarm clocks, why not 10 instead? 

Solution : 

By setting the snooze time to 9 minutes, the alarm clock only needs to watch the last digit of the time. So, if you hit snooze at 6.45, the alarm goes off again when the last digit equals 4. They couldn't make it 10 minutes, otherwise the alarm would go off right away, or it would take more circuitry.