Where are the battery capacity files located?
I think the answer in the email is legit, here's why:

All of the following information is based on the Google Nexus 5, but the general procedure should apply for all Android phones.

The mail describes two possible sources for an app developer to get the information on how high the battery charge is on full capcity:


Reading from the sysfs file in the Android file system (/sys/devices/power_supply/battery/charge_full_design).
Using the Android private API. Private in this case means that only system apps (from Google) have access to this information. But an app developer can use a few tricks (described here) to get access to this information too.


Both of these methods have their own source for the "charge at full capacity" value.

1.: In the Nexus 5 this file is created by the driver responsible for handling the chip (Maxim MAX17048) which measures the remaining capcity on the battery. The source code of this driver can be found here. The driver uses additional files to get device specific information. This information is fixed and usually provided by the smartphone manufacturer. The file used for the driver in the Nexus 5 also contains a value called fcc-mah. You will get the content of this value if you read from the /sys/devices/power_supply/battery/charge_full_design file in the Nexus 5.

2.: Another way is provided by the private Android API. This API uses power profiles to know how much energy every component approximately consumes. The Google documentation says:


  Device manufacturers must provide a component power profile that defines the current consumption value for the component and the approximate battery drain caused by the component over time. This profile is defined in platform/frameworks/base/core/res/res/xml/power_profile.xml. For guidance on these settings, see Power Values.


The power_profile.xml also holds a value called battery.capacity which can be read via the API. This value also holds the maximum capacity of the battery.

To get the power_profile.xml for your device, do the following:


Use ADB to get the the framework-res.apk: adb pull /system/framework/framework-res.apk
Follow these instructions to get a readable version of power_profile.xml
Path to the extracted file is: framework-res/res/xml/power_profile.xml




So depending on how a developer chooses to get the "charge at full capacity" value, he may get different results. Especially when I look at the driver file for the Huawei Honor 6, one can see that there are different values depending on the used battery type. 

One could assume that a lazy manufacturer does not update the files in the driver or the power profile appropriately for the used battery, which in return causes the different values.

Form a developers perspective the second method is more attractive and easier to implement (again, see this answer). The first method needs access to the Android file system and root privileges.