Check Schedler again there you see "+ 30 seconds" and you get option to sellect Hour and Minutes only.
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It's because of the schedule offset at the org level:
A small amount of time that is added to schedule start times to help prevent server overload at standard schedule start times. An organization has a single schedule offset that is applied to all schedules. The schedule offset does not affect the start time of manually started tasks or taskflows. You cannot change the schedule offset.
Even though it is not displayed in the schedule details, the schedule offset for your organization is added to the time range configured for all schedules. This ensures that scheduled tasks run as often as expected. For example, you configure a schedule to run every hour from 8:00 a.m. to 12:00 p.m., and the schedule offset for your organization is 15 seconds. Your schedule runs at 8:00:15, 9:00:15, 10:00:15, 11:00:15, and 12:00:15.