I think you have another assumption which is wrong.

"Miles" only ever means real physical miles (at airlines) on the earning side, and only with real airline miles programs (and these days, only with a few of the airlines any more). Back when earning real physical miles was common, you earned one mile for each mile you flew.

But redemption is never by physical miles (that you have), in any program.

With airline programs that have the closest thing what you might call "real" miles, you earn 1 mile for every airline mile you fly, but then you have redemptions such as within the continental 48 states starts at 25000 miles, to Hawaii starts at 35000 miles, etc, etc, and has nothing to do with the cash cost of the flight.

Now, in those airline programs, you can only book according to the chart when there is award "availability", which is often not the case. (If it's not "available" at the above "saver" booking, you may have to use tremendously more miles.)

So that's the background of Capital One. Capital One heard that some people were complaining about how "hard" it was to use their airline miles, and they said, here, we have "miles" which are much easier to use. But they tried to hide the fact that the valuation was completely different than the airlines. Because if you want to use a "bad" valuation, most US-based airlines will always have availability at some redemption valuation, just often not a good one. And that's exactly how it also is with Capital One "miles", if the flight is expensive on cash, it's expensive with Cap One "miles" also, if the flight is cheap with Cap One "miles", that's because it would have been cheap to buy also.

So Cap One "miles" are very different than airline miles, sometimes better, sometimes worse, but all of Cap One's advertising only focuses on the cases where they are better, and ignores the fact that sometime real airline miles are better.

The thing you need to understand is that there is no industry definition of "miles". Any company can create their own "miles" currency and make it work however they want.

Capital One basically makes cashback cards that can only be redeemed for travel, and then tries to pretend that they are somehow related to airline "miles" card by using the term "miles". But it's nothing more than a cashback card. Signup bonus aside, you'll earn way more cashback (that you can use for travel or anything else) with a 2% cashback card like Citi DoubleCash than with a card that only earns (effectively) 1.25% cashback like the one you have from Cap One.

To not get these rude surprises, you have to understand that all credit cards that do not have the name of a specific airline or a specific hotel are simply giving you their own cashback-equivalent currency, and you have to compare the redemption rules and restrictions against a cashback card. And rarely to do they work out any better (again, other the signup bonus, perhaps) than a 2% cashback card would. (And Citi DoubleCash has no annual fee, so why not get it instead of any card the earns less than 2%, if all you're interested in is cashback?)