Montr, the problem is that some dealers interpret the def refill only with scheduled service, so if the DEF requires refill outside of a servicing cycle, it's on the customer's on tab. Other dealers will top the car off until the next servicing period. When the DEF is serviced during a scheduled event, the DEF tanks are purged, and filled completely with new fluid, vice simply being topped off.
On my last servicing, I had been doing notably more city driving and did need to top off my DEF prior to the scheduled service.
2016 GLE 350d - White on Black