@szdavid: You’re crossposting (asking the same question multiple times in different places), which is not too well received in many forums. See Dataview, strange display.
Short answer: The output is perfectly alright: You asked for area grouping and got it (right column). One Group “-”, one group “Databases”, one group “Lecture”. The elements of these groups are shown in the left column.
It will do that for all cases where it doesn’t see area:, just tried to verify. So you should check if, say “Prochains livres” actually contains an area: in the front matter (maybe not there, typo, empty or wrong data type).