XPS 15 the battery when at 20% drop immediately at 6%

Hello, I have a XPS 9550 laptop that have almost a year of life. In the recent weeks the battery when arrive at 20% of charge suddently drop to 5% without a good reason (during play a mp4 or during write with MS Word for example, CPU is costant and there are no heavy operation).

The diagnostic test is passed (with elapsed time of 00:00:00 in report, is it normal?) and the battery health is at 88%, it have only 33 charge cycles because, when I can, I usually use the laptop with the charger.

I've writed at Dell Suppot (I have the premium warrantly for another two weeks), I sent the requested screen of the battery informations from the bios and executed the istructions, the answer was that battery is ok and all is normal (in this screen the bios report that battery status is "Excellent").

This is a graph of the battery report generated from Windows when you can see the last drop from almost 20% to 6% (is also reported that design capacity is 84 mWh and full charge capacity is 74 mWh):

edit: I trying a Battery Calibration with full discharge