Calendar alerted me at strange time

My iPhone calendar alerted me at 12:00 o'clock and then again at 1:20 which was strange because the event was for 12 o'clock. When I went to the events settings it said Alert at "Time of Event" and it had a second alert at "Tuesday March 19 at 1:20" this was so strange. I don't have an option on any of my other events to alert me at "Tuesday March 19 at 1:20." It's either "5 minutes before," "15 minutes before," "30 minutes before," etc. Was this just a glitch or maybe was it because I had the event stored for yesterday and then switched it to today? Weird iPhone.