buffer times not correct

I'm using Zoom Scheduler through Chrome on a Mac. 

I am trying to set up 30-minute meetings with a 15-minute buffer at the beginning and a 15-minute buffer at the end. I am available from 11:00 AM until 3:00 PM. 
I have input this information into Zoom Scheduler, but it is showing the meeting times available as 11:30, 12:00, 12:30, 1:00, 1:30, 2:00, and 2:30. 

I was expecting the first meeting time would be listed as 11:15, since that would give me the requested 15-minute buffer between 11:00 (when I become available) and 11:15 (when the meeting would start). I was expecting the last time available would be listed as 2:15: that meeting would end at 2:45, giving me the requested 15-minute buffer before I am no longer available (at 3:00). 

What am I doing wrong?