Contradiction comes from their sources probably! I don't even know how they can do the math these days, except legal gambling sites and casinos you have illegal ones, and we have crypto gambling! To gather accurate information from all of them is impossible!
Therefore, these stats are just for legal fiat casinos! We can only speculate how big crypto gambling is, and how many illegal gambling activities there are across the world, I doubt that anyone can provide accurate info about that.
Taking into account that gambling is not legal everywhere then we can only speculate how big it is, also while gambling could be legal in many countries it is heavily regulated and even in those countries there is a lot of illegal gambling going on, so as you say it is impossible to know how big the industry really is, however estimations can be made and it is obvious that in the countries that are more prosperous the gambling industry must be bigger as people have more money to spend.