I am sure some can find more dimensions.
1: is the setting private or in public?
2: who is the person having a conversation with the royal?
3: is the culture in a country very hierarchical or more laissez faire?
Let us imaging the Crown Prince of Denmark is having a private conversion with a good friend. In this situation I can easily see the friend refer to Queen Margrethe as "your mother".
And his you address royals have changed slightly over the years.
You have probably also seen contemporary public speeches by royals addressing their parents as mother, father, mummy etc - and not Your Majesty...
All in all - I think it is very hard to compare how Queen Mary acted to how people act now a day. That is the problem when you find old examples and compare them with modern day situations... because how people and even royals interact with each others have changed over time.
Another example is curtsying. If you see televised events where guests are introduced to e.g. the Queen of Denmark - you will see that some do not even courtesy to the Queen. Before the gala dinner of the 50th anniversary of Queen Margrethe ascending to the trone - you could see the Prime Minister of Denmark not curtseying to the Queen - when the Prime Minister was introduced to the Queen...