I, and my son, both own an AT Pro. There's variation in both machines on the same target. Different swing angles will change it too, as will ground mineralization.
For example, on the same quarter, mine can ring up 87, and his 86 or even 85. I think it has something to do with the differences in FIRMWARE loaded on the equipment. Firmware tweaks change the operating behavior slightly (usually for improvements, but a slight tweak can change the VDI numbers by 1 or 2 points, too.). At my last servicing, my AT Pro got a firmware upgrade, it stabilized the VDI numbers, but also seemed to raise them by about a half-point on average. My son's equipment (bought used, but many years old) is definitely running much older firmware.
Mineralization in the ground can change how targets ring up by a SIGNIFICANT amount. In some of my parks, quarters consistently ring up 87-88 consistently. A park in an older part of town (I have NO idea what is different about the dirt), quarters will ring up at 84, consistently. The key here... is consistently. it's always a good idea to dig a couple dozen targets, regardless of the ID, when you show up to a new site, to ensure you're getting a read on how targets behave in the soil you're detecting over.
Bottom line, firmware differences can account for VDI differences. You might have an AT Pro that "matches" the VDI output of an AT Max, or you might have one that behaves slightly differently. Also, small hardware variations, can change how the signal is processed, too... as a software/firmware QA manager for 7 years (last job), this is very apparent to me.
Cheers!
Skippy