Current measurements can be varied becasue of many factors
1) Its hard to 'measure' capacity correctly. Capacity is basically maths = current x voltage x time, However practically this isnt so simple.
a) When charging, the current and voltage values change. These include physical disturbances, to electrical performance and output of the charger.
b) Whats Most important is how the charger mathematically calculates capacity, and how often it measures it. if it only multiplies current and voltage every couple of seconds, there is lots of room for error.
c) power may be also be lost in the cables/contacts due to resistance. This applies for all chargers but essentially requires good sound electrical design, including what the user connects the batteries with. Thin coil spring terminals are notoriously bad under load.
These which come to mind now, are all dependent on the mechanical and electrical design of the charger (money) and the software design (more money in development). As with all things, the first to go with a tighter budget include achieving the absolute accuracy, while retaining the basic apparent functionality.