uwp the item matrix is not updating the correct value

If you want the value of ItemMargin property to change when the Page.SizeChanged event handler is triggered and then the value of ItemsMargin in binding source changes, you need to let the SampleViewModel implement InotifyPropertyChanged interface which notifies clients that a property value has changed, and apply the OneWay mode in {x:Bind} extension.

For example:

public class SampleViewModel:INotifyPropertyChanged
{
    private int itemM;
    public int ItemsMargin
    {
        get { return itemM; }
        set 
        {
            if (itemM != value)
            {
                itemM = value;
                NotifyPropertyChanged();
            }
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
    public void UpdateItemsMargin(double actualWidth)
    {
        this.ItemsMargin = (int)actualWidth / 2;
    }
}

Apply Mode=OneWay in the binding of ItemMargin property.

ItemMargin="{x:Bind ViewModel.ItemsMargin,Mode=OneWay}"