To the best of my knowledge neither royal nor sarkozy have insisted that pensions would remain the same, or be augmented. Since basic interest doesn't explain the vote, I would resort to some more symbolic explainations... but i'd be on shaky grounds.

My guess is that Sarkozy appeals to older people's desire for "law and order" -- for strength, for sécurité.

My sense is that many older people are afraid and have lost faith in the public authorities to keep them safe.  And I bet Sarkozy is much more reassuring for them than Royal.

Truth unfolds in time through a communal process.