The figure used on this page isn't the number of people who self-identify as "Black", it's the estimated number of people with some degree of African ancestry... in Latin America in general and Venezuela in particular these are very different things - "black" only applies to people who have sole or predominately African ancestry, while people of mixed ancestry generally consider themselves "Brown" ("Moreno") on the census (presumably this is what you are calling "mestizo"). If we simply used the "Black" figure from the census we'd be ignoring the large number of people who have mixed African and European/Native American ancestry. We can't use the "Brown" figure either, as this would include many people who have both European and Native American ancestry but no African ancestry. So the census figures are no good for us and we need to find a different source that estimates African ancestry directly without using the "black"/"brown" colour labels. The 20-30% estimation comes from an official document from the Venezuelan government (the Embassy in the US) which specifically states this as the number of people with African ancestry, so is a reliable source for our purposes. If you have an equal or better reliable source that gives an alternative figure then please let me know what it is. 
The 100% issue is a red herring - "mestizo" and "African ancestry" are overlapping categories and can't be added together as if they were distinct... "mestizo" in Venezuela means a mixture of European, Native American and/or African ancestry, so some of the people included in "mestizo" are also included in "African ancestry" and you'd be counting them twice if you added the two together. A better check is that we'd expect "Afican ancestry" to be all of the "Black" category plus part of the "Brown" category, so more than 3% and less than 53% - the 20-30% estimate fits this perfectly.