Men Grand Prix Series And FIRA 7s Division A/B

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Despite no information in Fira site, http://www.lyonseven.fr/ already said it will be played by 3 groups of 4, but it is not clear how it will go from there.
It makes no sens - as it is said - that for the second day 1st and 2nd of each group, plus 3rd of Group A and B will play the Cup.(http://www.lyonseven.fr/competition.php)
Why 3rd of groups A and B?? What about 3rd of group C?
Should be the best and second best third of the 3 groups...