It's because you're looking at a date/time field. The issue statistics gadget is pretty much useless when you're using fields that are not an option list.
In your case, if your filter had say 100 issues returned, you'd end up with around 100 groups containing one single item each. If your users were doing bulk updates or imports, and you used a larger data set, you'd probably get a few groups with the same minute and a handful of issues in them rather than a singleton.
I try to clarify
If there would be field "UPDATEBY" like in jql then I woul dket updated issues by user.
In JQL I managed to do that, but I want to see all users updated issues in last 7 days grouped by user
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You'll need to find or write an app to do that report, or write a listener or scripted field to pull that information from the issue history in a way that the existing reports can use.
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