From looking at the Rain Rate it looks like it was still raining. The Event is only off by 0.01 inches. The system that does the tally for the Event needs to see the rain register before it can add it to the Event. It just hadn't added it yet as it is happening right then as it is still raining. I hope that makes sense. Just wait for it to stop raining and the Event total should catch up. It could also be a rounding discrepancy because of the internal conversion of millimeters to inches.
Ambient's definition for Rain Event:
Rain event is defined as continuous rain, and resets to zero if accumulated rainfall is less than 1 mm (0.039 in) in a 24 hour period. Some weather stations may calculate this slightly differently and the customer should reference the User Manual.
