Why are Alkaline, NiMH, and Li-ion AA measured differently (mAh, mWh, etc)?
It's marketing, like PMPO watts. Big numbers sell!

If the battery is 1.2V nominal, there won't be a big difference between W.h and A.h numbers.

But if the battery is 3.6V nominal, the W.h number will be 3.6 times more bigger than the A.h number! So a 1200 mWh 3.6V battery sounds a lot better than a 330 mAh battery... even though it's the same battery of course, I bet one description sells more product than the other...

It's like ebay LEDs. 20000 millicandelas, wow! (and one lumen).

Any battery can be measured in W.h or A.h.

A.h is more relevant if the load draws constant current.

W.h is more relevant if the load draws constant power (for example, it uses a switching DC-DC).

Alkalines typically aren't rated in mAh because 1) the internal resistance is a bit high so capacity decreases at high load current. So a single capacity number would only be correct at a certain load current. And 2) since pretty much all alkaline batteries are the same, how the hell would they sell you "brand name" for 5x the price of "low end" if you knew they had the same capacity?.......

Also there is no 18650 cell on the market with more than about 4000mAh capacity, and usually it's around 3000-3600, so all the "9999mAh" cells are counterfeit. They are basically "whatever we found and could slap a label on." There are tons of counterfeit batteries for sale, especially lithium. My personal opinion is that if someone lies on the capacity, I'm not going to trust them to sell a safe product either.