In reality it's never been simple to measure the combat power of a fleet. Early in the 20th century there was an explosion of new ship types (subs, destroyers, battleships, aircraft carriers, fast attack boats armed with torpedoes, and so on) and it's still unclear how one should measure all of these new warship types against each other. For several centuries before that it was pretty much a matter of counting the number of heavy guns you had afloat. But for the last century there's been a lot more variety, and measuring the overall strength has become difficult. It only became worse with the introduction of effective anti-aircraft and anti-ship missiles in the last half century, as well as the introduction of "smart" (software controlled) mines and torpedoes in the last few decades.

These days fleet comparisons are best shown on a spreadsheet, with not one overall value but several (capabilities for landing troops, delivering air strikes, sinking enemy merchant and warships, clearing mines, and keeping hostile subs from your shipping). Most fleets specialize and a few (like the United States) are pretty good across the board. But politicians and the media prefer just one number, even if it's misleading. Simplicity rules in journalism and politics.