Some questions for the record and the manual.
Does the Combat Power calculation just add current in-the-field land and naval strength? Garrison and fortress troops (other than auto-garrisons)? Any factoring in of reserves that could be mobilized?
Why do the Combat Power numbers that appear when I boot up a save change dramatically (indicating my country has more power) if I save it and reopen it later? What is updated in the calculation?
What determines the countries that appear on the list?
My Draft Take on This: These are the Great Powers plus countries with which you are at war (though China may pop up as well if it exceeds a Great Power. It seems like some priority is on proximity and lower relations.
Some have commented that the nation comparison omits countries and they would like a scrollable list of every significant country and its stats. For the 19th C, however, that may be too much information - military intelligence was spotty and often rudimentary. So the current list works for its purpose.