Failed appointment scheduling

I originally scheduled an internet installation appointment for May 24. When I scheduled it, I confirmed on the phone that I could change the appointment date if necessary. On May 9th, I wanted to change the appointment date and was directed to the xfinity assistant to do so. I chatted with an agent, and asked whether there was a availability to change my appointment from May 24th to May 17th. They said that there was availability, and confirmed a new time for me on May 17th. On May 15th, I realized I never got a confirmation email for that change. I once again chatted with an agent. They said they had no record of my May 17th appointment. I asked again to reschedule my May 24th appointment to May 17th. They confirmed that someone would call me on May 17th 30 minutes before arrival. I confirmed again with the agent in the chat that I had an appointment for May 17th.

On May 16th, I called Xfinity again to confirm, and spoke to an agent who said I had no May 17th appointment, and that there was not any availability on May 17th.

Why did two separate live agents in the live agent chat falsely assure me that I had a May 17th appointment?