Weekly and monthly views show different values

We've recently noticed that in the Worklog [User - Day wise] report, the reported hours for the exact same dates seem to be different in weekly and monthly views. For example, check 27-Sep-2018 on the following screenshots: Last week - no logged work is visble for any of the users: last_week Last month - logged work is visible for all three users: last_month

The users are the same and are in the same order on the two images. The screenshots were taken a few seconds apart.