Hi, Kevin,
I think that your interpretation of the analysis shown on the label of the dry and canned petfoods does not take into account the moisture content, which skews the total fat and protein %age figures. In both the examples you give, the RATIO of fat to protein is quite similar.
What you need to calculate is the TOTAL fat and protein fed - mutiplty weight of food consumed by the % fat and protein content.
Older, less active dogs do require less fat in their diet, and since many older dogs have a degree of kidney impairment the protein content of the diet is better reduced, though the quality of the protein content should remain high.
The reputable petfood manufacturers take all these factors into acount when formulating there senior diets, which are to be recommended. Remember that in all cases, especially when a dry diet is given pleant of clean drinking water should always be available.
I hope this helps.
Walter