John
I think that you might be comparing apples to oranges. From what I've seen Dutch Ovens and Camping/Hearth Ovens have two different sizing systems. Dutch Ovens seem to follow the Skillet sizing system. A #8 skillet is usually around 10" across. A #8 Dutch Oven is usually around 10" across. Usually with the same size DOs and Skillets, the lids will even fit each other, or be real close.
The Camp Ovens I've seen are usually marked with how many inches it is across the piece. Some even say for example 12 IN on the lid, or in the case of the Camp Spider Skillets, on the handle. Your #10 Camp/Hearth Oven should be around 10" across, same as the #8 Dutch Oven Lid is, as it is also around 10" across. To add some confusion, Lodge seems to measures their CO's across the top, where it is widest. Griswold seems to measure the bottom of the oven where it's narrow. I bet if you measure across that #10 you'll find it's 10".
Found my Lodge Catalog:
#8 Skillet=10-1/4" Diameter
#8 Dutch Oven=10-1/4" Diameter
#10 Camp Oven=10" Diameter