Those are unusually low numbers. Did the Ukrainians take a day off? Were the artillery crew preoccupied watching Wimbledon? Or is the Russian army literally out of material?
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I don't have a source available but there is a media blackout at the front last reports indicated some gains in north of bakhmut and Russian forces appear to be lacking reserves to shore up there lines according to pro ua sources I last checked.
This could be causing low numbers but also slow days happen at the front. Logistics and moving the rear closer
As you're saying, it could be so many reasons - or nothing. It's just a single day. Let's see tomorrow or even the next 5 days to interpret anything into the numbers.
Sundays are typically slow days. It isn't clear if Ukrainian is not pushing as hard (there are numerous reasons they might not), or just that the people reporting/collecting numbers are not working (meaning numbers the rest of the week are inflated as they catch up )
We all deserve a day of slacking off from time to time.
Hangover day
400 casualties per day is about average.