Calendar appointment off an hour

We have a situation where we have a superintendent and then two secretaries with access to his calendar.  Secretary 1 created an appointment for 10:00 AM Eastern. But when the Superintendent and Secretary 2 look at his appointment from their device it shows as 11:00 AM.

1.Checked time zones in both the desktop outlook and office.outlook.com and it was set for Eastern.

2.Checked the time zone on the appointment on all three users and the time zone is eastern. 

3.It is only this one appointment and not any other appointment.

4.When I busy search the availability of my superintendent is shows as 11:00 to 12:00.

We cannot see how this is happening. If it was all appointments it may be a time zone issue, but it is only one. 

Any suggestions?