I don't think it's what "dealers are doing" - it's that dealer, and it's incorrect. You've now added in enough oil for it to be at the correct level and, as you said everything is fine. You now know more about this topic than that dealer, which would make me wonder how much else he doesn't know. Just say'n......
I’m new to this but, not new to vehicle maintenance. It seems like Triumph dealers are tightly controlled by corporate. Apparently, they are required to use and sell expensive Triumph branded parts and materials. I’m not willing to say I know more about the subject than my dealer—after all I’m new to this bike—but there is a clearly stated volume of oil type and quantity to use when replacing oil and filter. That is 4.6 liters. I believe my dealer is doing that. I think there is a difference at the corporate level because a) the oil level doesn’t matter much on a dry sump engine and b) they want to control costs for the dealers. I do not believe there is any sinister intent by my dealer.
I gotta change my oil in about 800 miles (2 weeks) since I do it every 2000, I'll add 4.6 liters and see where it brings the oil level, I'll post what I actually put in after that because I know it takes more usually. Hermies oil change kit comes with 6 liters of oil so I'd guess I use over 5 liters.
I have a bad memory, but I seem to recall needing to add about a liter of oil after putting the recommended amount. just to get it in the cross hatch. I'll pay more attention at the next change after this thread.