They correspond to the larger eras in French economy.
Industrial revolution
Entre-deux-guerres, a period of strong urbanization and a huge push towards social housing. I suppose they included WW2 cause nothing was built there anyway.
1946 to 1970 is “les trente glorieuses”, the time of rebuilding everything, which means everyone had a job and could afford a house or apartment.
The oil crash in 1973 ushered in a more modern era, usually more left-wing after May 68 and with the election of Mitterrand in 1982.
The 1990 one is around when we elected a right-wing president and the public policies vastly changed.
2005-2006 was starting to get tough because of oil again, I believe. It is also around the beginning of the US subprime crisis, of which the consequences affected us all too.
They correspond to the larger eras in French economy.
Then they should be labeled to reflect that.