When a copy job starts a new "interval" it will wait for a restore point to appear and then start a copy.
Let's take this scenario:
Backup job normally starts at 12am and finished at 2am
Backup Copy Job interval is set for 3am.
Here's one case:
On 6/1/2017 the backup job scheduled for 12am did not run for some reason (say it failed for some reason)
On 6/1/2017 at 3am the Backup Copy Job interval starts and the job looks for a new restore point. Since there is no restore point, the copy job waits
On 6/2/2017 at 12am the Backup job starts
On 6/2/2017 at 2am the Backup Job finishes
On 6/2/2017 at 2am, the Backup Copy that started at 6/1/2017 at 3am now job starts copying
On 6/2/2017 at 3am, the copy interval ends, if the copy job is not done within that short 1hour between 2am and 3am, it will stop the existing copy and fail with an error that the copy interval expired.
If you really need to have a gap between the end of the backup job and the start of the copy job, what you should do is change the copy interval to start at 12am and then change the schedule to prevent the copy job from transferring data between 12am and 3am.