Here's an interesting reasoning: https://covid.joinzoe.com/post/uk-cases-stop-rising
As I understand it, it goes something like: the list of symptoms which are required in UK for you to be able to get a free test is pretty much limited to cough or loss of smell or taste, but Delta has a different symptoms profile, so the more people get Delta the less the proportion of those who gets tested.
I see people wonder about what happened in the Netherlands. And though drop might have been accelerated by school holidays, seems like a lot of people are not aware about what was the reason for the sharp rise on the prior week. From locals' perspective it went like this: the government allowed night clubs to open, requiring patrons to either be [fully? don't remember] vaccinated or show recent negative test. Pretty quickly some vaccinated asymptomatic positives infected a lot of unvaccinated negatives and the thing was spreading like wildfire among 20- an...
Well, not all. Night clubs are closed, you can't drink past 10 PM, there are restrictions on number of people at events. Plus masks stay mandatory in a lot of places.