Yes, and that is totally fine. Consider this example with 3 events in 2 hours:
16:00 field=123
16:01 field=234
17:01 field=123
If you do a dc(field) for first hour, you get 2 as result, because you have 2 different values for field. If you do a dc(field) for the the second hour alone, you get 1 as result, because there is only one value of course. that doesn't mean though, that you get 3 as result if you do a dc(field) for the whole time. The result is 2, there are still only two distinct values for field. So having the sum of dc() first hour and dc() second hour which is 3 is different than the dc() over the whole time range. That is perfectly fine.
Your example tells us that 9 field values happened both in the first and the second hour, the rest were distinct for each hour.
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