Different phones use different voltages for their batteries, yet manufacturers market their capacities in milliamphours. This makes comparing batteries across phones* difficult and a little confusing. Was this on purpose? Is there a good reason for it?
*I know that different phones have different power draws, even if they had the exact same batteries.
Batteries in general have always been rated in mAh. This isn’t anything specific to cell phones. This usually hasn’t been an issue and it allows you to easily compare two different batteries of the same size. For example an el-cheapo AA battery’s mAh rating is probably going to be a bit under 1000 mAh, where a good alkaline might be up somewhere around 2500 to 3000 mAh. There has rarely been any need to compare the total energy between two batteries of different voltages as you end up comparing different circuits and it is easier to calculate out the actual draw of each circuit and compare the total hours of use that way.
As you noted, different phones have different power draws, which would make any mWh comparison rather inaccurate.
What I mean is like let’s say you have an SLR camera, which can take either two lithium-ion battery packs or several AA batteries. The packs and the AAs run at different voltages (at least on paper) and so you have to do the math to figure out which gets you more hypothetical runtime.
Similarly, when comparing two laptops using similar chipsets, you might able to estimate 20-50W of average power draw on both devices, but comparing their batteries are very difficult unless you can get the Wh ratings on both. Many eBay sellers, for example, only list amp-hours without the voltages.
Well its actually important to get the mAh accurate , so as to avoid overcharging them.