How long time does it take to charge your Lumia?
Mine takes about 75 minutes from 0% to 100%. When I check diagnostic it is charging with aproxomatly 550-600 mA. In 75 minutes it will only be able to charge around 750 mAto the battery when charging 600 mA for 75 minutes. The battery should be 1300 mA with means that it could not be fully charged.
Is Diagnostic fooling us or is there something strange here?
Your calculation is correct except that units are not properly stated. 600mA for 75 mins is 750mAh (600mA x 1.25 hour) so it should not be able to charge a 1300mAh battery. 75 mins to fully charge a battery from 0% looks too short so you may be right that the Diagnostic is inaccurate.