0-100 KM/H times as reported by Dragy far off Audi's official claimed times

Dear fellow A8 drivers,

With reference to the topic matter, please allow me to quickly introduce myself and provide some background info. 

My name is Pieter, and in 2019 I became the lucky owner of a mint 2018 A8 50 TDI Pro-Line Plus (more or less full option). A couple of months after purchase, I took the car to ABT, who installed their tuning box, upping horsepower to 330 HP and torque to 650 NM. Now recently, I got myself this Dragy GPS device, which supposedly can measure 0-100 times (a.o) quite accurately. This weekend, I performed several runs, but was unable to clock a time faster than 7,1 seconds, which is quite a bit removed from Audi's claimed time of 5,9 seconds, and keep in mind that's the quoted time for the standard, non ABT-tuned vehicle ! Now granted, the A8 sits on winter tires (Goodyear Ultra Grip M+S), so I would assume that this could be the culprit, but would that really make that much of a difference ? If so, how much do you reckon it could (on or about) go faster on quality summer tires, like Pirellis ?

Looking forward to your replies, and many thanks in advance !

Best,


Pieter