AbstractBackEndHierarchicalDataProvider: getOffset() always returns '0'

Vaadin 6 / 7 / 8
1

Hello!
I have an implementation for AbstractBackEndHierarchicalDataProvider:

public class MyDataProvider extends AbstractBackEndHierarchicalDataProvider<MyEntity, Void> { 

    @Override
    public int getChildCount(HierarchicalQuery<MyEntity, Void> query) {
        MyEntity parent = query.getParent();
        
        int count = 0;
        if (parent instanceof MyEntityChild) {
            return 0;
        } else if (parent instanceof MyEntityParent) {
            count = getMyEntityChildCount(parent); 
        } else {
            count = getMyEntityParentCount();
        }
       return count;
    }

    @Override
    protected Stream<MyEntity> fetchChildrenFromBackEnd(HierarchicalQuery<MyEntity, Void> query) {
        MyEntity parent = query.getParent();

        if (parent instanceof MyEntityChild) {
            return Stream.empty();
        } else if (parent instanceof MyEntityParent) {
            return fetchChildren(parent, query.getOffset(), query.getLimit()).stream();
        } else {
            return fetchParents(query.getOffset(), query.getLimit()).stream();
        }
    }

    @Override
    public boolean hasChildren(MyEntity item) {
        return getChildCount(new HierarchicalQuery<>(null, item)) > 0 ? true : false;
    }

    ...
}

Trouble:
While scrolling, the methods getOffset and getLimit() always return the same values:
getOffset() = 0, getLimit() = getChildCount(…)
I always get all the data from the backend
Why getOffset() always returns zero?
Thanks