Hello Anthony, 

- The TOTALS formula calculates the total number of times an event was triggered.
- The UNIQUES formula calcultates how many unique users triggered the event.
Using “historical count = 1st”, we could expect no differences between the two.

My guesses are the following: a unique user could be using several devices to trigger the event before being identified as a single unique user by Amplitude (by logging in on the several devices they used for example). This difference could be either the sum of these “historical count = 1st” on different devices, a user launching the event several times quasi-simultaneously, or any other chronological issue happening somewhere in the tracking and definition of the “historical count” property. 

You can also visit the Amplitude help page describing how “Historical count” is calculated, which states that ‘Amplitude's definition of "Nth time" is itself time-limited’. This could be another pist to explore.

These are my guesses and I am unsure that they are correct. I encounter a similar difference using my population. Hope I could be of some help. 

Thanks for this helpful response @Matrak. 

@Anthony Please keep us posted as you make progress. Were you able to achieve the results that you need? We can’t wait to hear more!

Thanks @Matrak @Jeremie Gluckman .

No, I still don’t have the answer to the question.

Anthony

Thanks for the update @Anthony. I can let the team know. Please keep us posted as you try things out and make progress. 

Thanks @Jeremie Gluckman 

It looks like it might happen sometimes when the same event happens in the same millisecond, that it sends 2 events instead of 1, and then it counts two “firsts.” 

Should we change our Android SDK implementation to prevent duplicates? The event below is triggered by a human tap on their Android device, so it’s shouldn’t be possible for a regular human to send this twice in the same millisecond. 

Amplitude customer support confirmed the following: 

Events with the same timestamp are counted multiple times for Historical Count = 1st when using “Total,” so it's usually more appropriate to use "Unique Users" rather than "Total" when using the Historical Count = uN] functionality.

Great @Anthony! Thanks for closing the loop here