Seems to be different specs for battery, Apple says.........
Battery and Power2
- Up to 10 hours wireless web
- Up to 10 hours iTunes movie playback
- Up to 30 days of standby time
- Built-in 83.6-watt-hour lithium-polymer battery
- 87W USB-C Power Adapter
The above is from here......https://support.apple.com/kb/SP794?locale=en_CA
The standby test measures battery life by allowing a system, connected to a wireless network and signed in to an iCloud account, to enter standby mode with Safari and Mail applications launched and all system settings left at default. Battery life varies by use and configuration.
The last sentence says it all ?