When setting your process execution schedule it is best to set the schedule to run at minute intervals that are easily divisible by 60. The reason being is that your process will execute at consistent intervals.
Some examples of this are highlighted in the reference guide article:Scheduling Dialog
However, there may be instances where you need to schedule a process that doesn't run on a schedule that is easily divisible by 60. For example if you need to schedule a process to run every 7 minutes 24x7x365 you will run into problems using the scheduler. The scheduler is designed to run at the set interval (in this case 7 minutes) but when it reaches the next hour it will start over from the beginning of the hour.
So if you are scheduling the process to run every 7 minutes you will have an instance at the end/beginning of every hour where your process will execute too early.
A workaround for this is to use an external scheduler. Design your process with a Web Services listener as the start shape. The external scheduler would send a request to the listener which would in turn kick off your process every 7 minutes without the restart at the beginning of the hour.