Values ​​change when grouping date changes

Content

We have developed a report that show how many tickets a particular Group has performed. For this we are using the Transactions table, and filtering by "assigned", that is every time a ticket has been assigned to a particular group it should post.

However we put a date filter of the transaction, and we are also creating a period grouping, but every time we change the Year / Month grouping to Year / Month / Day the results change considerably, even within the same period.

I would like to understand why these values ​​change when we change the grouping.

The definition of the report is attached.