Are all dark chocolates with the same amount of cocoa equal?
There's just a ton of different types of dark chocolates out there that cater to all kinds of needs:


Chocolatiers
Bakers
Gourmets
...


To answer your questions: 

Yes, it's rather easy to have a "more fluid" dark chocolate that still contains a high % of pure cocoa and cocoa mass by adding more cocoa butter.

Cocoa mass is a raw ingredient that comes straight out of the beans and cocoa powder and cocoa butter are the separated products from cocoa mass, so to add more sugar, Chocolate producers use the same amount of cocoa mass and powder but a bit less cocoa butter.

One additional factor to consider is the chocolate bean roast and quality. Like coffee, the roasting process can affect the taste.