Upvote:6
The 1997-2006 flag (your third flag) is the first flag (1960-1963) of independent Congo, then known as Congo-LΓ©opoldville. While it's true that it's design is reminiscent of the Belgian Congo flag, I imagine the symbolism of being the first flag of independent Congo was a major factor in choosing it over any other design.
The 1963-1971 design (your first flag) was adopted when a seventh province was created. The six stars at the left represented the first six provinces of the new state, and the Congolese decided they didn't want to add a new star every time they created a province.
Interestingly, the 1963-1971 flag was also used from 1997 to 2006, as the presidential flag. From 2006 and onwards it's the official flag of the country, replacing the 1960-1963 / 1997-2006 design.