Wrong order of properties in Edit mode

Hi,

Even though I use "Order" when decorating my properties they don't show up in that order in Edit mode. How can I fix this? The code below is one of my properties and I have like seven more with Order = 20, Order = 30, Order = 40 and so on...

[Display(
   GroupName = SystemTabNames.Content,
   Order = 10)]
[UIHint(UIHint.Image)]
public virtual ContentReference BannerImage { get; set; }

Thank you!