I found this study interesting. In Germany you have must-Children, who normally turn 6 at the end of the period like end of June or August, depending where you live. Can-Children are the ones who turn 6 later in that year but deemed ready and Delay-children are the ones who are normally old enough but deemed not ready and can go a year later.
In order to delay you normally need a letter from your nursery, virtually all children attend it, and a paediatrician, depending on the local authority they do it easy or they make it difficult for you. It also depends how many children have to go in a year and how many are delayed from the previous one or applied to go early.
Interestingly lots of boys are delayed compared to girls.
Saying that, Bavaria is a state which is normally very keen to get them in, delaying is very difficult and even my friend who moved from England to Germany with a child not 100% secure in German, 9 weeks premature and showing it and a birthday just before the cut-off day had no chance, she was told he has to go.
Also a child finishing the first year in primary is at the equivalent of Y2 at least. It is a very different path and generally totally structure, no play with daily homework So I would assume that delayed children, being a year older, are deemed more ready and teacher may expect them to be more advanced. Maybe these children are so used to the nursery surroundings, school is such a bigger shock and they can't cope.