I wonder if it has something to do with the locus it's based in. In the US, Speckled Sussex are based on e+/e+ (wild type) but I believe Jubilee Orpingtons are typically based in E^Wh/E^Wh (wheaten). Wheaten allows for good expression of red but not as much black. Similar to how I had a brown-based Red Orpington, I wonder if you had wild type-based Jubilees you could add consistently
I could be wrong however, but just a theory!
