Seems an odd setup. Why would the sub-panel have a 100A disconnect at the main panel, and another 100A breaker at the sub-panel?
Quite normal actually.
The 100 amp breaker at the sub panel acts as a disconnect for the panel, which would be required if the sub panel were at a detached building. Also, in the case of a manufactured home, many require (via their listing) a disconnect even when the service panel is part of the same structure.
Thanks JP. This was a manufactured home so that explains it.
Main breaker panels are cheaper than main lug panels in many cases.