Having Googled it, it seems that it has to do with the car's weight. Smaller and lighter cars can meet EU6 emissions standards without the AdBlue system.
You can pop your reg into a ulez checker and find out. You might not live near London or a city that is clamping down on diesels, I was just surprised that there’d be a 2017 MB that was non ulez compliant
Regarding the "ULEZ checker" websites, they are not always accurate. When I bought my car I input the details on the TFL official website (they operate ULEZ I think) and it showed that my car was not ULEZ exempt and I would have to pay the charge if travelling in the zone.
When I challenged this, they informed me that I would need to provide them with proof from MBUK and a copy of V5 showing Euro 6.
I had to get some paperwork from MBUK and send it off to DVLA and then got an updated V5 and forward to TFL who then updated their database to show my car as ULEZ exempt. Phew.
Having Googled it, it seems that it has to do with the car's weight. Smaller and lighter cars can meet EU6 emissions standards without the AdBlue system.