UMP having an absolute majority in parliament is a new phenomenon ;  it used to be that the right wing was 2/3 - 1/3 RPR and UDF. There has always been a UDF-UMP majority in the senate, and it evolves very slowly.

Since it's a proportional indirect election though, the left wing always have a fair share of the senate, unlike in the parliament.

Also, the recent tendency is that after an election the RPR-UMP wouldn't give support to UDF members not rallied to the president's party ; thus in the Assemblée Nationale the deputies had a strong incentive to go to the UMP, whereas senators (with their 9 years mandate, some were elected in 1998... )didn't have this incentive and remained in the center right party.

Un roi sans divertissement est un homme plein de misères