Look at the first item in the "WrapPanel" (first image). How does its "size" compare to all the items that follow it? Is it bigger or smaller?
In the absence of an explicit "item" Height and / or Width, the "first" item (size) in the collection drives the "desired size" for all following items.
So, insure all items are the same size; or, the big ones come first; or, using specific Widths and / or Heights.