I use Battery Doc and mine shows 1700mAh capacity. Maybe the battery apps just aren't that accurate and simply round to the nearest hundredth ¯\_(ツ)_/¯
Components, especially batteries, don't exist in a vacuum. There are normal variations, not every SoC will perform the same or have the same memory read latency etc. The spec may be 1624 mah, but a battery that is 1600 mah won't get thrown away.
I purposely avoid running apps like this. I'm happy with the general performance of the battery, I'd rather not obsess over the details to the point where I'm not longer enjoying the phone.
Apps like this will cause some to obsess over a small differences in capacity. These indicated capacities can change daily so no need is sweating this.
I wouldn't worry about it...some of these apps may not know how to measure the new phones accurately. After John Gruber posted his iPhone SE review, it seemed the CPU speeds Geekbench was reporting for his phone was about 300MHz less than mine. A few reboots and I got even different numbers. Either way, if the batteries are showing the same thing, it's close enough not to worry.