I set up a new basic testing environement using following scripts to create index and mapping, and import datas.
https://www.wetransfer.com/downloads/0e2cb9930e37a56a7504ba70160e295d20160606123941/9e3c84
Data file contains 324 events which concerns only the 2th of may 2016.
As you can see my new index contains 324 documents.
The mapping has following structure:
Events are displayed in Discover tab:
I obtain 324 results when I agregate my data by date histogram on ACT_ACTIVITY_TIME field.
But if I choose to set my metric agregation type to unique count I obtains 335 results...
Note that I obtain differents results depending to the field I choose.