Kennet; Based on what you’ve indicated, here’s what I think is happening.
When you specify week by week, project looks at the overall weekly allocation. While we people think that might result in 8, 8, 8, 8, 8 hours each day, in reality, it could end up with 16, 16, 0, 8, 0. On the weekly level, the resource is not over allocated because it did not exceed 40 hours. But there are days where the resource is allocated double and days where the resource is not allocated at all.
When you select day by day, project makes certain the daily allocations don’t exceed 8 hours. So the previous example of 16, 16, 0, 8, 0 becomes 8, 8, 8, 8, 8. In the broad scope of things, both end up with the same result being 40 hours per week.